Page 159 of the manual:
"Store
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.
Sign in to an account: Choose Store and tap Sign in, then enter your user name and
password.
View your iTunes Store account information: Choose Store and tap View Account,
then type your password and follow the onscreen instructions.
Sign in to a different account: Choose Store and tap Sign out, then tap Sign in and
enter your username and password."
http://manuals.info.apple.com/enUS/iPhone_UserGuide.pdf