how do i replace air pistons on hatch?

Visit your local auto parts store and purchase the pistons. There are several different types of mounting methods for these gas-charged lift supports, but every one I have ever bought has had detailed instructions printed right on the package to show you exactly how the various methods work so you can identify what kind you have, and how to install the new ones. It's an easy job requiring only pliers and perhaps a flat-bladed screwdriver, and can be done in about 15 minutes. Be careful when working on the Jeep hatch, as they're pretty heavy. You'll want to replace one piston at a time, and use a board or pole to support the hatch while you do this, so you don't drop it on yourself.

Umm...are you really asking this? You shouldn't ever work on a car if you have this question. Go to auto parts store. Tell them what you need. pick up part. take off old part. put on new part. EASY...ISN'T IT?