You can upgrade your Lumia 520 to Windows 10 mobile making some changes to the registry (you need to interop unlock your phone).
Warning: Windows 10 mobile works
very slow in Nokia Lumia 520 and the phone get very hot. I had to increase Page File to increase a little bit the speed.
Also, when you update to Windows 10 mobile you can get more updates, and if you install they you will get more updates of Windows 10 mobile. I think latest update you will get is to Windows 10 mobile is 1607, but latest updates break bluetooth and bright control.
I used
this method and it worked to get Windows 10 mobile update, but be careful and don't do this in your main phone.