Sometimes it doesn’t work upgrading or downgrading via the UniFi Controller GUI. If that is the case, doing it via the CLI is quite safe and should work without any issues.

Below is a step-by-step guide:

• Download desired firmware (i.e v4.3.23.4913544)
• Run FileZilla Client
  
• Use sftp://yourfwipaddress with Siteusername and Sitepassword (leave Port blank), then Quickconnect
  
• Upload the downloaded firmware upgrade.tar to the connected folder
• Run PuTTY, Terminal (Mac/Linux) or any SSH-client to connect to your firewall
• Connect using credentials Siteusername and Sitepassword to yourfwipaddress
  
• Make sure you are in your homefolder (i.e /home/admin if your Siteusername is Admin)
• Run sudo syswrapper.sh upgrade upgrade.tar
  
• Once fully run device will restart and you will get disconnected from the session
• Reconnect via browser and fill in the correct WAN IP details if needed
• Run PuTTY, Terminal (Mac/Linux) or any SSH-client to connect to your firewall
• Connect using credentials Siteusername and Sitepassword to yourfwipaddress
  
• Set the inform address again by running set-inform http://yourcontrolleraddress:8080/inform
  
• Device will now reconnect to the Controller and Provision with the right settings
• Done!

Note: This manual procedure can be used regardless if you’re upgrading or downgrading.

Good luck!

Source: Instructions for down or upgrading Ubiquiti (UniFi) USG