some pop up unscrew from the top try holding back under neath and turn the top part

some have it where the rod that pushes up and down on the pop up comes out and and the nut unscrews holding the pop up up to the sink and drops down

and some times as plumbers we get that pop up that just wont come off no matter what we do and we have to cut it off

if you have to cut it off try to drop the nut that goes up to the sink as low as it will go then cut the pop up tube where the water goes down we use a electric saw but this can be done with a hack saw ( please note this method can get aggravating but take a break every now and then and you will get it )

now if the nut that hold it up wont budge you have to cut the nut this i recomend you buy a close quarter saw and cut the nut once you are 3/4 of the way through put the tip of a screw driver in it and light tap it in you willl hear the nut snap then undo it with a wrench

just be carfull you dont want to ruin the sink or yout fingers

its just one of thoes jobs