I can’t update app

See if iTunes Support can help :

- go to http://www.apple.com/support/itunes/ww

- click on your country's flag (the country where you live, and which is therefore the country on your account)

- click on Contact Support towards the top right

- click on Get Help under 'Talk To Us'

Try logging out of the account in the app, close it via the phone's taskbar and reboot the phone, and then retry

What is the text of the message/prompt that you are getting ?