From the
Windows 8.1 Preview FAQ - Microsoft Windows
- If you're currently using Windows RT, you won't be able to uninstall Windows RT 8.1 Preview. You'll be able to upgrade to the final edition of Windows RT 8.1 when it becomes available.
The only way back to 8.0 is from a Recovery Disk from an 8.0 Surface RT. If you have a MS store nearby, you might want to pay them a visit. Take your Surface RT, at least a 4GB thumb drive and ask politely if you can make a 8.0 recovery disk from an existing Surface RT 8.0. They
might let you... but no guaranties.
Then boot off the 8.0 thumb drive and refresh your Surface RT.