Go to PC Settings > Update & recovery > For developers . Select "Install from any app (side-loading)." (This alone doesn't jailbreak, but it's required).
Method 1: Windows RT 8.1 Development Tool (Software Exploit) jailbreak windows rt 8.1 surface
To enable "Test Mode" or modify kernel memory to accept unsigned ARM-compiled programs. Limitation: Go to PC Settings > Update & recovery > For developers
: