There is a specific pusher tool used for Sig sights. Many target shooting facilities, gunsmiths and gun store have them. Some may let you use the tool for free or charge $10 and do it for you. A punch will ruin the gun, so don't even attempt it.
I used a plastic mallet & a small wooden dowel. This worked fine for me when I installed a set of Mepros on my 220ST but having a matte stainless finish is much harder to hurt. I have also used the wooden dowel on a od-green Springfield finish with fine results. Just don't get in a hurry or slip, and be sure to only tap the dowel against the sight & not the slide. I am fairly brave though, you may want to use a pusher if one's available to you. :wink: