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'

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

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