How to change iOS ID?

When Julia was visiting me she signed into her Facebook using Janet's iPAD .. Janet has loged into her Facebook since and thus Julia has been logged out. However, everytime Janet selects a picture to upload to FB it uploads to Julia's FB page even though she is not signed in..

Janet has tried deleting the FB app and re-installing it but the app still remembers Julia's account.

How does Janet delete Julia's FB login/account or change the iOS account to mine...

Janet