Can't update apps in iOS App Store; can only Open
I had same problem and fixed it by closing the app down then reopening it, selecting the updates tab, then quickly press on 'update all' before the app updates revert back to 'open'.. You only have a split second to do this but it did work
Force quit the App Store (open the App Store, then hold power until the slide to power off slider appears, then hold home until the app quits), then reopen it and go to the Updates tab. Let it refresh the list and the problem should be resolved.