Microsoft has stopped seeding windows 10 mobile test builds to non supported Lumia phones but there is an unofficial way that will give you windows 10 mobile officially. This method basically hacks your phone into a Lumia 950 phone that will push latest official version of windows 10 mobile.
However it is different from the way we gave for forcing your windows 10 mobike to Redstone build as it involves unlocking bootloader, rooting windows 10 mobile, changing registry to hack it to Lumia 950 and then unrooting, locking bootloader of your windows phone again. This method works for old Lumia phones like Lumia 520, 630, 635, 720, 820, 920, 925, 1020.
Step by step instructions on how to do download official windows 10 mobile on non supported Lumia windows phones
2. For sideloading using XAP application deployment tool, you may need a developer account and may need to use the “Windows Phone Developer Registration” application to developer unlock your mobile. For my Lumia 630, I used the “CustomPFD Registry Editor + Tweak” version 0.6 alpha.
3. Using that registry editor, I went to Registry Tree -> HKLM -> System -> Platform -> DeviceTargetingInfo and changed the values as following:
PhoneManufacturer value to “MicrosoftMDG”
PhoneManufacturerModelName to RM-1085_113025
PhoneHardwareVariant to RM-1085
PhoneModelName to Lumia 950
4. Then fire up WP internals to disable root and also restore the original bootloader. For your information, once I enabled the root and unlock bootloader after doing a windows device software recovery and the next update to CYAN didn’t happened, I received an error after downloading the update as it can open the file (8007000d). Hence we are restoring the bootloader and root status.) Now I even opened the Windows Phone Developer Registration application in PC and unregistered from Developer status.
5. After this, I restarted the phone and saw the new update that had started downloaded. This was version 10.0.10586.107 of windows 10 mobile. Then I downloaded windows insider application from the windows store and opted for fast ring since I wanted the latest version. This put to 10.0.14393.5 which is latest at the time of this writing.
Let me know in the comment section below if the aforementioned hack worked for you. It is the only way to save our windows phone 8.1 device from programmed obsolesce.