I wondered the same about a Lumia 520, then found that if it can be done at all, it can only be done with difficulty, and little certainty of success. If I recall right, it requires bootloader unlocking, and changing the phone's identity to fool the update server into thinking that it qualifies for a W10 upgrade. It's probably a breach of the TOS, and from what I've read, not worth bothering with.
Edit. I've just come across Conan143's magnificent Guide about this sort of thing:
[Guide] Installing Redstone on Unsupported devices (X2X Series)