So the Lenovo has left the W520 off its Windows 10 compatibility list and this has surprised many as this is one of their premium laptops. Some are not even able to get Windows 10 to load. One of the reasons is that Nvidia Optimus implementation on the Lenovo W520 sucks.
However the main reason is that Intel decided to support only Ivy bridge and later platforms for Windows 10 so Lenovo had no choice.
That said, you can install Windows 10 yourself. For this, go into Bios and disable switchable graphics and Wireless LAN and these two are said to cause installation to fail.
Win 10 upgrade program has improved by a long mile and it will tell you to remove Rescue and Recovery for installation to continue. After that Windows 10 will install and run just fine.