It's simple really, but it just takes a while to do this.
Two solutions:
1. You can go on amazon and search up "wii u lcd screen" and find the one by Nintendo. Under it is tools that are INTENDED for the repair.
'''"Warning: You NEED to buy the tri-wing screwdriver, and if you don't have a BIG size array of phillips head screwdrivers, buy the one there.."'''
2. Just go to your local Target or Nintendo store, and buy a new GamePad.
Both solutions work. 1st one takes alot of time an risk, 2nd one is easy, yet pricey.
(And I'm not talking about gas prices..)