Each iPhone has a specific iTunes account attached to it, which is used to sync music, videos and other purchased content to the device. In order to change your iTunes account on your iPhone you'll need to log in to iTunes on your computer and sync your iPhone to iTunes. Once synced, the new iTunes account will be synced to the iPhone and will allow you to make purchases under the account user's name while in the App Store and iTunes application on your phone.
Open the Applications folder in Finder and double-click the "iTunes" icon to run iTunes on a Macintosh computer. For PCs, click "Start," "All Programs," "Apple" and "iTunes to start the program.
Click the "iTunes Store" tab on the iTunes sidebar, and click the "Sign In" button.
Enter the new iTunes account e-mail address and password, and click the "Sign In" button.
Insert the USB cable into your iPhone's dock connector on the bottom of the phone, and plug the other end of the cable into your computer's USB port.
Click the "iPhone" tab in the iTunes sidebar and click "Sync."
Disconnect the iPhone once the "Safe to disconnect" message appears.