Hi Brian,
If this AP is already registered with a WLC then you can change it via WLC.
In WLC GUI, go to Wireless -> Select the AP -> Credential -> Override Global Credential-> Update username/pw/enable pw
In WLC CLI, you can configure it like this. If it is for all APs, then select all, otherwise AP name.
(WLC3) >config ap mgmtuser add username admin password Cisco123 secret Cisco123 ?
all Applies the configuration to every AP that does not have a specific user name.
Enter the name of the Cisco AP.
(WLC3) >config ap mgmtuser add username password secret
(WLC3) >save config
Are you sure you want to save? (y/n) y
If it is Autonomous AP you can configure using the "username xxxx password xxx" IOS command
HTH
Rasika
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