I have been struggling with this too. Changed it on my Mac but could not change it on my Iphone. After half an hour I realised that all I needed to do was click on the wrong apple ID and sign out. Then when you sign in again use the new details. I may have misunderstood the problem but this was not obvious to me and I could not change it for love nor money!
On your Mac/PC open iTunes (make sure it is updated). On the tabs on the left click on "iTunes Store". This will change the window on the right to the iTunes store. The top right corner should have your Apple ID or Sign In written. If there is an Apple ID written click on it and Sign Out. The corner should then change to "Sign In".
Now click on Sign In and sign in with the wrong/incorrect Apple ID that you no longer wish to use or update from.
Once logged in, click on the down arrow that shows when you move the pointer on the top right corner where it should now show the apple id, and select Account.
The page that opens up should have a section called "iTunes in the Cloud" with a tab on the side saying "Manage Devices".
Click on Manage Devices. Its should list your iPhone or iPad that you no longer wish to use with this Apple ID. And click Remove.
Once its confirmed click "Done" and from the top right corner select Sign Out.
Been having this problem on my ipad for awhile now. Ever since they made me switch from short userid to full email userid. I'm logged in (have logged in and out repeatedly to make sure) under the current email userid and pass but trying to update apps (even ones ive downloaded SINCE d/ling them with new userid, I STILL get the old non email userid pop up asking for the password to that. Absolutely no way to change userid am i going to have to redownload and set up and reinput ALL my information EVERY TIME ANY APP has an update?????
<Edited by Host>