Not really that difficult (as long as the new hatch is the same size).
Remove the screws from the outside (open the hatch to expose the heads). On my O’Day 322, they are screws, not through-bolted.
Pry up the old hatch (carefully), clean up the old sealant, mask off the area outside of where you want to seal the new hatch, apply a good bead of sealant (sika flex or similar), place new hatch over the hole, and screw it down.
When you pull the old hatch out, look at the core in your roof top…mine was foam…at least around the hatch.
If yours is balsa, you may want to make sure it is dry…and maybe seal it with epoxy.
There may also be a trim ring on the inside of the cabin. I took mine down, but I don’t think I had to. The screws on the inside may have little plastic covers on them.
Good luck…shouldn’t be too bad.
Greg