How to Upgrade an iPhone to OS4
- 1). Install iTunes on your computer from apple.com/itunes/download.
- 2). Connect your iPhone to your computer using the USB cable that came with your phone. ITunes launches automatically after a few moments.
- 3). Click the name of your iPhone, such as "Brandon's iPhone," in the menu on the left.
- 4). Click "Check for Updates." A prompt will appear asking if you want to upgrade to the latest version of iOS 4.
- 5). Click "Yes" and follow the on-screen wizard to complete the upgrade process.
Source...