There are a number of threads where I have already explained this.
Read my earlier post about updating OTA, then you understand.
Follow this:
- In iTunes uncheck "sync this iPhone through WiFi" and uncheck "iCloud...", click "Apply".
- Disconnect the iPhone, to be sure do the hard reset: Hold the Sleep/wake button and the Home button at the same time and keep holding them, ignore everything on the screen, keep holding them until you see the white Apple logo.
- Quit iTunes.
- Download the iOS5.0.1 file directly (~700MB) and leave it in the download folder (make sure you download the exact file for your iPhone).
- Start iTunes.
- Connect the iPhone with the cable.
- In iTunes "Summary" page, Alt+Click on the "Restore" button, choose the downloaded iOS5.0.1 file. Restore.
- After the restore is complete, Alt+Click on the iPhone icon on the left in iTunes, click "restore from backup", then choose a backup from before you updated your iPhone, this brings back your data.
- After you are satisfied that everything has gone correct, copy the downloaded iOS5.0.1 file to the following folder youraccount/Library/iTunes/iPhone Software Updates. Keep the downloaded file for a few days, until you are sure that everything stays right.