Download the firmware .bin files from:

• The Downloads page under the Firmware section, or
• The Releases section in Community.

Upgrade the device firmware from the device Properties window.

• In the Devices tab click on the device to expand the Properties panel.
• Select the Config tab → Manage Device → Custom Upgrade.

https://help.ui.com/hc/en-us/articles/204910064-UniFi-Upgrade-the-Firmware-of-a-UniFi-Device