They are thin wire clips that snap around the base of the ball. Unsnap them and remove. It's hard to describe in words here. I can see the clip in your photo so they are there. Just mess with it, you will figure it out. It's easy.
You'll see where the end of the wire is wrapped around the strut at the base of the ball. The wire has the end bent back to create a rounded tip. Pop the loop off of the shaft, then slide the pin straight out the end. The ball should come off the mount if you lift the weight of the hatch off the strut and give it a yank. Thump the new one on and replace the pin (if your replacements have the same style retainer).
Sometimes they're a pain even with the clips off. If you ever have to pry them just get a long screwdriver and stick the handle between them and the strut and hit the screwdriver end to pop them off while someone holds the hatch down some. They're harder to remove fully extended than they are compressed some.