How do you remove and existing drain from a tub. Mine leaks and needs replaced.

You use "dumbell" tool to remove the bottom part. You use screwdriver to remove overflow plate and lever.

Now, if you want to replace entire drain, including rough-in, then you have to get behind the tub and replace entire assembly. It is kind of difficult project to do for average HomeOwner.

I agree with Milo, tough job for most. Easiest way for many is to get at it from below ebven if that requires removal of a ceiling below.