The update is not yet available for the Lumia 520 ( or Lumia 521) in any North American, South American, or Middle East markets. The update is expected to become available in these markets shortly.
As with the Lumia 1020, 925, 920, 820, and 625 the Lumia Black software update, which ships on top of the Windows Phone 8 Update 3 (GDR3) software update, includes enhancements for camera performance, changes to a number of system settings, and enablers for a number of new Nokia apps. However, because of its more limited hardware, the Lumia 520 does not include some features, such as Glance Screen 2.0 and any app requiring the device to have 1GB or more of RAM (e.g. Nokia Beamer and Refocus).
On the plus side, early reports indicates that the Lumia 520, in common with the Lumia 625, gains the "double tap to unlock" feature as part of the Black update.
More information on the new features and updates is available in our comprehensive guide to what's new in the Windows Phone Update 3 and Lumia Black software updates.
For more information on the exact availability for your Lumia 520, visit this Nokia page, which lists availability information for each device variant.
As we have previously noted:
The list is divided into countries and sub divided into device and operator variants. The country variant is the generic (SIM-free) product for the country in question (i.e. devices without operator lock and/or branding). Each variant will show one of three status messages: "available", which means the update is ready for download, "coming soon", which means the update has been approved and will shortly become available, and "waiting for approval", which means the update is undergoing check with Nokia and/or operators.
Once the update is available for your device, you will be automatically notified that the update is ready to download and install. This notification process relies on the device carrying out a weekly-or-so check for new updates. You can manually force an update check from Settings -> Phone update.
After the updates has been downloaded, the installation process will take between 20 and 45 minutes. This will include several devices reboots. The installation process requires a certain amount of free space in the internal memory. The exact amount varies, but is usually at least 1GB. If enough space is not available the install may fail with an error message (e.g. "cannot open update").
After installation, you may find that you actually gain free space overall, since any previously orphaned files created under Amber are found and deleted.