I have a 1991 F150 4x4. I am having trouble getting the wipers off to replace them. Not just replacing the blades I am replacing the whole wiper. I can't get the old ones off can someone please help????

Raise the wiper up. On the backside of the wiper arm, you will see a small tab. Use a screwdriver to pull this tab all the way out. Let wiper arm back down. It should stay up off windshield, and pull arm straight up.