I have looked at it for a friend once and just looked at the hatch on my 5th gen for a quick reminder.
The first thing you will notice when you remove the spoiler is that 4 bolt holes will be still be in the rear hatch. That is enough reason for me to leave the spoiler. And on the 5th gen, your rear wiper assembly is under the spoiler.
Now, if you still want to remove it and the wiper assembly is not there.
1. Open the hatch and notice 4 plastic grommets along the top inside edge of the door. Remove those grommets.
2. Look inside those holes and you will see a nut, I think a 10 mm or 12mm socket will remove it. Remove those nuts.
3. Now the spoiler should come off the hatch. You may need to disconnect the wiring for the rear wiper assembly.
4. Plug the hole on the outside of the hatch. You don't want water getting inside the door when you wash the truck or it rains.