wrong apple ID on wife's iPhone

After upgrading my wife's 3GS phone to IOS5 i accidently entered my apple ID on her phone and now all my contacts and such get put on her phone as well as mine. How can i get her phone to change back to her apple ID?? forgive me if this is somewhat of a dumb question but Ive tried everything I can think of. It recognizes her phone and ID on iTunes but when I need to do something like update an app, it shows up as my ID. Thanks for any help.