cant update apps on my iphone as the id userame is an old one..how do i change it to the on that works?

ta

Apps need to be updated using the Apple ID that originally bought/installed them.

I get this a lot cos the missus has a separate iTunes account but we both share the apps cos we sync to the same iTunes library. When one of the apps she bought needs updating I have to type her password into my phone. Bit of a pain.

If you don’t know the password I think you are screwed. Best bet, if it is a free app, would be to delete it then re-install it with your current Apple ID.

Yep no way to combine aps to one account, best replace if free aps as Graham already explained