They ususally last a long time. If you want to change make sure you put a 2x4 and snug it tight against the top so the tailgate won't fall on your head. Take needle nose pliers and pull the retaining ring around the top ball while you jiggle the liftgate, the strut should slide off.
Let the door rest on the 2x4. Do the same on the bottom ball and remove strut. Before you install new one put a little dap of grease on ball (sometimes the new strut comes already greased).
On the new strut all you have to do is push until it snaps on. Do the same on the bottom. Then do everything the same on the other side. Just make sure the board is secured - the liftgate is heavy.