Use Store settings to change or create an iTunes Store account. By default, the iTunes account you’re signed in to when you sync iPhone with your computer appears in Store settings. You can change accounts on iPhone to purchase music or applications from another account. If you don’t have an iTunes account, you can create one in Store settings.
Choose Store and tap Sign in, then enter your user name and password.
Choose Store and tap View Account, then type your password and follow the onscreen instructions.
Choose Store and tap Sign out, then tap Sign in and enter your username and password.
Choose Store and tap Create New Account, then follow the onscreen instructions.