Have you tried to reset your App Store? It works for me most of the time. If that fails, Do a RAM reset. If you not sure how to reset let me know.

I recommend you try App Store reset first.
Open your App Store as you usually do.
Tap 10 times at the bottom of the "task" bar on one these (Featured, Catergories, Top Charts....etc).
After 10 times the app screen will sort of blink.
Try updating again.
If the above doesn't solve it then do a RAM reset (aka force restart)
Press & hold the home button & on/off simultaneously UNTIL the Apple logo reappear.*

*When you press and hold the buttons the screen will go blank, at this stage do NOT release any of the buttons. Just keep holding until the Apple logo appears.
Sometimes the RAM reset may not solve the problem immediately but from my experiences 2 or 3 days later everything is ok again.