If you want to upgrade a lightweight AP then just connect it to a controller with the required firmware .I'm guessing you meant autonomous? If so follow this - https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/107911-ios-...
That command will switch it to the other device. Just confirm that the HA link cable between the two is up and running etc so that they can see each other as there was a reason that they failed over in the first place. Check the logs.
If you add the ...
Off the top of my head it sounds like the HA Pair has failed over to the secondary and you cannot install licenses to this device. Fail it back to the primary and then try adding the licence.
Double-check the configuration between the WLCs. All the WLCs in the mobility group need WLANs in the same order (WLAN ID). Also make sure that the AP Groups are identical. It happens due to the configuration not being completely the same even though...
