Procedure to Manually Update iOS 4 For Mac OS X
- Connect your iPhone or iPod Touch to your Mac
- Start iTunes if it hasn’t started automatically.
- From iTunes click on your iPhone or iPod touch from the device list
- Hold down the Options key and click on Update button
- Now browse and select the downloaded iOS 4 firmware update file and click on Open
- iTunes will do the rest of job to update the firmware to iOS 4.0.
It will may take some time for iTunes to complete the updating of your iDevice firmware to iOS 4. During this process do not disconnect your iDevice until the update is completed.