1- Get the latest version of iTunes to your computer;
Download iTunes for Mac (Free)
Download iTunes for Windows (Free)
2- Launch iTunes and connect your device to your computer;
3- Select your device in the Source List on the left, under Devices;
4- At this point, iTunes should detect that iOS 4.0.1 is available for your “supported” device. If not, hit the “Check for Update” button.
5- iTunes will notify you that a new version of the iPhone operating system is available for download. Accept it by clicking “Download and Install;”
6- Allow iTunes to perform the software update for your device.
Important: do not disconnect your device until the update has finished! You will know that the update has been successfully applied when the device reboots and shows the iOS 4 home screen.
Alternately, owners of a supported device can download the iOS 4.0.1 update / restore IPSW file to their computer and install at a later time. Direct download links from Apple’s servers are available by going here – iOS 4.0.1 IPSW Download Sources.
When are ready to update, users can launch iTunes with their device connected, select their device, click “Restore” with ALT (option key) held down, select the iOS 4 IPSW firmware bundle, and click “Choose.” The installation process will begin, while the same golden rule applies – do not disconnect until the update is finished.
