Assuming you initially upgraded from Windows 8.1 to the 10130 Insider build less than a month ago and you have not performed a full disk clean up to remove previous Windows installations, you should be able to restore your previous Windows 8.1 installation by navigating to "Settings App > Update & Security > Recovery > Go back to Windows 8.1".
Please help, it's my only computer!!![]()
Even if it's a OEM license, you can reinstall Windows 8.1 and activate it by phone. To get the phone activation dialog you need to disable your internet connection.
That's why I asked.
I don't know if you can use any ISO image and put your OEM key to work. Usually you will need an OEM image. And you should had done a Recovery Image before put an preview OS on your machine.
If you manage to install your 8.1 you can activate with your key through the phone just like Marcos said.