A Unifi UAP with firmware older than v4 won't upgrade using the usual method via the Unifi controller.

You can attempt to cache the firmware on the controller first or run the following command via SSH. The credentials will be ubnt/ubnt if it's been factory reset first.  

Note that the URL in the command below uses http: and not https:. Ensure that the URL you use for this command includes http:, not https:.

The links here are examples for UAP v4.0.80 and can be replaced by newer model specific versions from https://ui.com/download/unifi 

CAUTION: DO NOT use this version firmware on a UAP-AC or other model that does not specifically use this firmware. Get the correct firmware link from the Ubiquiti download page first.

upgrade http://dl.ui.com/unifi/firmware/BZ2/

A UAP with firmware of v2.x won't upgrade using the above v3 method since the upgrade script doesn't exist in that firmware version. Run this command below first and then the one above after a reboot.  If the UAP has been adopted and provisioned the SSH credentials can be found at the bottom of the site settings page once advanced features have been enabled and saved.

wget http://dl.ui.com/unifi/firmware/BZ2/ -O /tmp/fwupdate.bin && fwupdate.real -m &


- The above commands require the UAP to have access to the internet.  That can be confirmed by pinging from the SSH console.

- The wget method may also be required for access points running version 3.7.x or below.

- If internet access is not available then you will need to download the firmware bin file, host it on a web server within the local network and adjust the URL in the commands accordingly.

- This process is also known to work with some other Unifi products such as Unifi switches.