The updater file is stored in your Home directory even after your iPhone or iPod touch has been updated. Specifically, it’s hiding in <Your Home Directory>/Library/iTunes/iPhone Software Updates, and is named iPhone1,2_4.0_8A293_Restore.ipsw
You can back up that file to another storage device if you want, like an external hard drive or CD. You can use the file to reinstall the upgrade later without first re-downloading the updater file. Here’s how:
- Launch iTunes.
- Connect your iPhone or iPod touch to your computer.
- Select your iPhone or iPod touch from the Library list on the left side of the iTunes window.
- Click the
Summarytab. - Option-click
Restore. - Navigate to
<Your Home Directory>/Library/iTunes/iPhone Software Updatesand select the iOS updater file.
Option-click Restore to select the iOS 4 update file you previously downloaded.
Once the process has completed, your iPhone or iPod touch will be ready to use again.
iOS 4 was released on June 21 as a free upgrade for the iPhone 3G and 3GS, as well as the iPod touch. For tips on preparing for the upgrade, check out TMO’s iOS 4 Upgrade Guide, and our Quick Tip on checking to see when the upgrade is available.