- 1). Tap on the App Store icon from the iPhone's homescreen.
- 2). Go to the "Featured" tab along the bottom of the screen.
- 3). Scroll to the very bottom of the page and click on the "Account" button.
- 4). Choose the "Sign Out" option from the dialog window that appears.
- 5). Tap on the "Sign In" button and then select the "Use Existing Account" option.
- 6). Enter your Apple ID username and password using the on-screen keyboard. These credentials are the same ones you use to purchase content from the iTunes Store.
- 7). Tap "Done."
Source...