You can push it either way. If you want to be sure, get your new sight and by hand, work on pushing it in from either side. If one side is looser, push towards that side.
I did my Mod.1 XDs with the slide in a bench vise, and a steel punch. Good smack to break it loose, then light taps to slide it out. Installed my new TruGlo the same way - light taps till lined up.