To go back you would need to download the 8.1 Recovery image, put it on USB and boot to the USB Recovery image.... there may be some extra hoops as well to Recover over W10 with 8.1Was you an Windows 10 insider?
This looks easier than downloading an ISO.
Did it restore to 8.1 first and then update to 10?
I'm thinking of doing this as someone was using my Surface pro3 and decided to install some bits and now it runs hot with the fan constantly going.
As I didn't set up system restore I can't seem to go back