Can the Windows Phone 8.1 update be un-installed?
The best thing you can do is to reset your Windows phone to roll back to Windows 8 OS. However, it'll permanently erase all your Apps and data stored on the Phone memory.
Good luck though!!
See the video -
Downgrade Windows Phone 8.1 to Windows Phone 8.0
Get the Windows Phone Developer Preview
After you update your phone to a preview version of the operating system, you can’t go back to a previous released version of the operating system.
i think this is the same for all Windows Phone Updates, if you have the original firmware then maybe you can flash it to go back to Windows Phone 8
you can flash with the availability ROM for Nokia Phone Using Nokia Software Recovery Tool.
or you can try this