For stability and daily use, keep with an unlocked filesystem.
Custom ROM work on this device generally falls into three categories: nokia lumia 520 custom rom work
The Lumia 520 officially supports only Windows Phone 8.1. However, the custom ROM community created tools to force-upgrade it to Windows 10 Mobile. For stability and daily use, keep with an
You must use WPinternals to unlock the bootloader, flash a custom UEFI, and then deploy a custom ROM — all while keeping the phone’s SB (Secure Boot) keys intact. For stability and daily use
For users wanting to stay within the Windows ecosystem, the "Windows 10 Mobile Group" provides tools to upgrade the unsupported 520 beyond its official limit.