Now that there’s a desktop version of Windows 10 designed to run on laptops and tablets with ARM processors, it’s unsurprising that a handful of hackers have found ways to shoehorn Windows 10 onto old smartphones that had originally shipped with Windows Mobile software.
But up until now it’s been pretty tough to follow along at home. Now it’s easier.
The folks at LumiaWOA have released a set of flashable ROMs for the Lumia 950 XL that let you replace Windows Mobile with Windows 10 in about 5 minutes.
There may be ROMs for other Lumia devices in the future.

Note that the process will wipe Windows Mobile, so this is probably something you only want to try on a device you’re not using as a phone anymore. And if something goes wrong, it’s entirely possible that you could lose data and/or leave your phone in an unbootable state. So proceed with caution.
But for the most part, here’s what you need:
If you’ve got all of those things, plus a compatible smartphone, then you can follow the LumiaWOA instructions to erase all data from your phone, flash the ROM, and then reboot into Windows 10.
Update: The instructions seem to have been removed, but you can find instructions for creating FFU firmware at gus33000.me, and/or view an archived version of the instructions at the Internet Archive.
via MSPowerUser and @gus33000
Looks like the link to the instructions is broken
You can find an archived copy via the Internet Archive:
https://web.archive.org/web/20190506144919/https://firmwares.releases.gus33000.me/instructions.html
But it looks like he may now be encouraging folks to make their own FFU images instead:
https://gus33000.me/2019/05/23/making-ffus-for-lumia-devices/
Looks like half a GPD Win…
Windows looks so cute on that little screen!