I have a Lumia 630 and I had installed technical preview of Windows 10 Mobile, but I formatted my phone and now it is back to Windows Phone 8.1. Can I install Windows 10 Mobile back? Lumia 630 is unsupported.
You can use Windows Insider app to upgrade to Windows 10 Mobile Technical Preview but you just can't get the stable version since your device is unsupported.
Try using Upgrade Advisor app to do it. It will check and tell you if your phone is windows 10 mobile compatible and if it is, it will instruct you through the installation.
It worked for me and it was quite simple so I hope it works for you as well.