Q: Uninstall iOS update

I'm sorry, but Apple does not provide a downgrade path for iOS. Because downgrading is unsupported by Apple we cannot discuss it on these forums, but you can use Google to find information on how to install an older version of iOS on any iDevice.

Try reinstalling it:

If You Are Locked Out Or Have Forgotten Your Passcode or Just Need to Restore Your Device

iTunes 10 for Mac- Update and restore software on iPod, iPhone, or iPad

A Complete Guide to Restore or Recover Your iDevice (if You Forget Your Passcode)

If you need to restore your device or ff you cannot remember the passcode, then you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and re-sync the data from the device (or restore from a backup). If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present. Refer to Updating and restoring iPhone, iPad and iPod touch software.

Try restoring the iOS device if backing up and erasing all content and settings doesn't resolve the issue. Using iTunes to restore iOS devices is part of standard isolation troubleshooting. Restoring your device will delete all data and content, including songs, videos, contacts, photos, and calendar information, and will restore all settings to their factory condition.

Before restoring your iOS device, Apple recommends that you either sync with iTunes to transfer any purchases you have made, or back up new data (data acquired after your last sync). If you have movie rentals on the device, see iTunes Store movie rental usage rights in the United States before restoring.

Follow these steps to restore your device:

     1. Verify that you are using the latest version of iTunes before attempting to update.

     2. Connect your device to your computer.

     3. Select your iPhone, iPad, or iPod touch when it appears in iTunes under Devices.

     4. Select the Summary tab.

     5. Select the Restore option.

     6. When prompted to back up your settings before restoring, select the

          Back Up option. If you have just backed up the device, it is not necessary

          to create another.

     7. Select the Restore option when iTunes prompts you (as long as you've backed up,

         you should not have to worry about restoring your iOS device).

     8. When the restore process has completed, the device restarts and displays the Apple

         logo while starting up:

           After a restore, the iOS device displays the "Connect to iTunes" screen. For updating

          to iOS 5 or later, follow the steps in the iOS Setup Assistant. For earlier versions of

          iOS, keep your device connected until the "Connect to iTunes" screen goes away or

          you see "iPhone is activated."

     9. The final step is to restore your device from a previous backup. If you do not have a

          backup to restore or have forgotten your restrictions passcode, then restore as New.