Windows RT will not update after reset
You can reset a Surface RT to factory settings by holding down Shift as the machine boots up. Make sure the machine is connected to power. As the Surface starts, the Choose an option screen will appear. Select Troubleshoot > Reset your PC. On the Reset your PC screen, select Next. Choose Just remove my files. The Fully clean the drive option is more secure but takes much longer.
After the reset is completed, you can do the usual set up tasks (create user, connect to wifi, etc). However, if you try to run Windows Update, you will get continual errors. You must therefore manually install a few updates to get started. Unfortunately, these are not easily available for download as Microsoft force you to use Windows Update to get them. These updates must be installed in the correct order otherwise you will be warned that The update is not applicable to your computer. Various sites on the Internet suggest starting with KB2919355, but this is wrong as KB2919442 is a prerequisite.
These are the direct download links:
After installing the above, you should be able to get all Windows updates using the usual Windows update control panel. As usual, it may take a couple of cycles of updates to complete. These may include firmware updates.