That varies from headphone to headphone... You usually don't have to glue pads on good quality headphones. On mine they just snap in. They are built with repairability in mind... That's not the case with every headphone though, even some high quality ones come glued. In that case I don't know what to use... You should first find out if you can buy replacement parts for your particular headphone.

I mean I just buy a good set of headphones once, take care of them and then replace the pads every few years when they start to feel like they're going bad, but they never get THAT bad unless I completely abuse them for like 5 years or something...