If you want to log in with a different account then tap on the id in Settings > iTunes & App Store and sign out and you can then log in with a different account. But if you use iTunes Match or automatic downloads or redownload an account's past purchases then you might tie your phone to that account for 90 days : http://support.apple.com/kb/HT4627
If you want to update the primary email address on the account then you should be able toby signing into it via Settings > iTunes & App Store
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