Eventually every bathtub is going to need some repairs and for some reason replacing the drain and popup seems to be one of the hardest jobs for people to accomplish.
The problem many people have is when the drain gets corroded and they can’t get it unscrewed but I have seen some youtube videos of people going crazy on these things only to end up calling in a plumber. One guy actually moved away .. well he was moving out anyway and maybe he was just trying to make everything work before he left. Unfortunately the job was too big for him that was pretty funny to watch.
The main thing about this job is understanding how the drain is removed.
It really is not difficult you just have to remember that you turn Clockwise to Tighten and Counter Clockwise to Loosen the drain just like any bolt.
The second hardest part is getting a tool that will let you tighten and loosen the drain without taring everything up. This is important if you have a fiberglass tub because you don’t want to slip with a ratchet and slam your tool into the side of the tub or you will be replacing it.
Finally you need to make sure that the new drain is sealed so it won’t leak. You will need some plumbers putty that is not included with the drain and you may need to replace a large rubber washer that sits between the bottom of the tub and the drain pipe. This washer comes with the part but it is not used on top of the tub where the drain you are replacing comes into contact with the tub it is used below the tub to seal the pipe.
Parts and Tools Needed to Replace your Bathtub Drain
You will need the following tools and parts to replace your bathtub drain
A replacement Drain (same kind or maybe upgrade)
Plumbers Putty
A pair of vice grips.
A pair of linesman’s pliers
A Bathtub Drain Wrench if you cant use pliers
A small drain snake or piece of wire
Paper towles
Replacing your bathtub drain
The first thing that you want to do when you are thinking about replacing your bathtub drain is inspect the popup and see if you can remove it by hand.
Standard popups screw into place and can be removed by turning them counter clockwise.
If the popup is frozen then you will need to use a small vice grip to remove it. Be careful with the vice grip and wrapping the vice grip with a bath towel is not a bad idea when wrenching it out so you do not slip and damage the bathtub surface.
Once you get the popup out you can look into the drain and see if the cross bars that the popup screws into are in tact. Because soap and cleaners are caustic over time they will corroded the metal. If the crossbars are 
corroded then you will need to purchase a special wrench at the supply store that can grip the walls of the drain. If they are mostly in tact then you can try to use a pair of linesmans pliers to remove it.
Insert the linesman’s pliers into the crossbars of the drain. Even if a portion of the crossbars are missing from corrosion the pliers will normally catch on what is remaining.
Using both hands on the pliers you want to twist the drain out in a counter clockwise motion.
Getting the drain started can be pretty difficult and you may find yourself kneeling in the bathtub using as much strength as you can to brake it free but it will brake free if you turn COUNTER CLOCKWISES meaning the opposite way the hands on your clock turn. heheh Like i said before i saw someone tightening a drain as hard as they possibly could wondering why it would not come out.. they are 
lucky they didn’t really damage their plumbing and tub.
After you get the drain free you want to carefully pull the remaining plumbers putty back from the opening so it doesn’t fall in. Its not extremely important that none of it gets in your drain its just not a good thing because it could cause restrictions.
Inspect the rubber washer that sits between the bottom of the tub and the drain pipe. If it is in good condition you can use it again. Replacing this rubber washer can be difficult so it is best to reuse it if you do not see damage.
If you must replace the rubber washer you can insert the handle end of a small screwdriver into the pipe and push down VERY LIGHTLY to give you enough room to pull the washer out and then insert the new one.
Using the screwdriver while replacing it you can slip the washer over the screwdriver that way it won’t get free from you and get lost in the void under the tub.
Remember that you do not want to push hard or much because the pipe you are pushing on is attached to your overflow pipe and if you damage it then you will end up cutting a hole in the wall behind the tub or in the floor to repair it. This is really not fun.
Now that the drain is out and the rubber washer has been checked and the opening around the drain has been cleaned from old plumbers putty you can prepare the new drain for insertion.
Grab a piece of plumbers putty and work it in your hand until it becomes flexible. You will need to squeeze the putty until the warmth of your hands make it flexible and the oils in the putty are distributed evenly and it is not crumbling in your hand.
Now roll the putty into a snake shape about 6 inches long and apply it to the bottom of the new drain around its lip.
Do not get plumbers putty on the threads of the drain. It is not needed for sealing and it will only cause you problems on inserting it and later if you ever need to remove it the threads could be locked pretty tight.
I like to flatten out the snake shape so this is easier to do. You want an even 1/8th to 1/4 inch thickness so the putty will take up any variations on the tub base and provide a good seal.
Once the putty is evenly applied around the lip of the drain you can insert your fingers into the drain and twist it into the pipe. Do this finger tight and then complete the job by inserting your linesman’s pliers into the drain and twisting the new drain in tight.
You want to see the plumbers putty squeeze out between the tub and the drain flange.
DO NOT CRANK THE DRAIN IN AS HARD AS YOU CAN…
Remember this is a Steel drain popup that is being inserted into a Plastic Pipe and if you turn too hard you will strip the threads and you could also damage the tub.
Checking how tight the drain is…
You want to put some pressure on the floor of the tub and make sure that the drain is in tight enough that standing in the tub near the drain won’t cause it to move or make noise. If you hear your drain making noise when you stand in the shower it means it is too loose and you need to tighten it probably a half turn more.
Again don’t over tighten.
Now that the drain is in place you can insert the popup.
After the popup has been screwed into place and you have cleaned the tub of remaining plumbers putty and any other debris you should close the drain and make sure there is a tight seal by filling the tub with a few inches of water.
Final Note
Changing your drain and popup is not the most difficult thing in the world to do but it does take a little muscle.
Remember clockwise to tighten and counter clockwise to loosen .. people who can’t understand that are the only ones that have problems.
You should watch your basement or lower floor ceiling for any leaks after performing this job for the first few showers.
If you hear the drain thumping or making noise when you walk in the tub to take a shower then the drain is lose and you just need to tighten it slightly.