If you own a phone that doesn't support the Windows 10 Mobile preview build 10536 but managed to download and install it anyway, you might be able to go back to Windows Phone 8.1 in the near future. Microsoft Windows Insider head Gabriel Aul has revealed that Microsoft is "working on repair options" for those un-supported Windows Phones
Aul mentioned this issue on his Twitter feed:
"We're working on repair options for the devices that were not supported but got the latest build. Mostly HTC 8X phones, but some BLU & other. This is a high priority for us to solve and have a team dedicated to it. We will let you know as soon as we have info on recovery."
In the meantime, users who see Windows 10 Mobile build 10536 on non-Lumia devices should not download and install it and they should also de-list that phone from the Insider Program until the issue is resolved.
Source: Gabriel Aul (Twitter)