How to replace sink drain stopper
You're going to have to get under the sink on this one. Once there reach behind the tailpiece coming out of the drain. There will be a knob attached to the tailpiece that has a lever running through it. Unscrew the knob and the lever which is what you see when you look down the drain will pull out. Simply drop the stopper in the drain and reassemble it running the lever through the hole in the bottom of the stopper stem.
Tighten the knob just enough so that it doesn't leak and when you open the drain the stopper stays up.