Make sure you always do a backup before any updates, especially if you plan to upgrade your existing installation.
It is also recommend to make a backup if you are on a controller version later than 5.4.11. Once you upgrade the firmware the controller will be on 5.4.11, so if you were on something different then you'll need to reinstall it and then restore your backup.
Using UniFi 4.8.X, Untable or Testing?
If you prefer to upgrade the firmware but still wish to continue using 4.8.X, create a backup of your controller prior to upgrading, upgrade the firmware, purge unifi, install 4.8.20 using the .deb file, then restore your controller backup during the setup wizard. This method will avoid the need to downgrade your controller, which isn't supported.
All system customizations are removed on firmware upgrade. This would include the UniFi VoIP controller, if you have it installed. Please make sure to backup your VoIP controller configuration, and any other customizations prior to firmware upgrade.
Changes in 0.6.0:
Bundle UniFi Controller 5.4.11. See HERE for controller release notes.