If, you haven’t updated your OS to this one then follow the link given below to update to latest OS for 8520 device.

https://www.blackberry.com/Downloads/entry.do?code=3210DDBEAA16948A702B6049B8D9A202

Please do it on your own risk only.

Installation instructions.

Step 1: Download desired OS to your PC from the above link.
Step 2: Install and complete the setup process to have the OS installed.
Step 3: Remember to delete vendor.xml file if your carrier the OS is not from your carrier.
Step 4: Do a comprehensive backup in case anything goes wrong.
Step 5: Connect the device to your PC and click on the Loader and let the process initiate. Loader can be found in the same folder as the vendor.xml file.
Step 6: Do not unplug the Bold during the process. Process may take around 30-60 minutes to complete.

Source: Crackberry