Open your UniFi Controller, click on the device, go to > Manage , and use the custom upgrade field to paste the URL or upload the file.
Ubiquiti has removed many legacy files from their official dl.ubnt.com repository. However, through official archives and community mirrors, you can still find the legitimate binaries. --- Upgrade Firmware Version 1-255 Ubnt Download
Thus, the full intended line might have been: Open your UniFi Controller, click on the device,