Make sure you always make a backup before any updates, especially if you plan to upgrade your existing installation.
Using a release other than the one mentioned?
If you prefer to upgrade the firmware but still wish to continue using the controller version you were previously on, make sure to create a backup of your controller prior to upgrading. Then upgrade the firmware, purge unifi, install the previous version 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.
UniFi Controller 5.10.20. SeeHERE for controller release notes.
*This release follows our usual release structure which means it will initially be available via this blog post only. It will be posted to the download site and official repos in the near future. If you aren't familiar with our release structure, please take a moment to read our post HERE. Thanks!