Bartholomew provided the link for creating a new user:
https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-change-the-username-and-password-fo....
In short:
SSH into your AP
Run the following commands:
>en
#conf t
#management-policy (if you haven't created a new policy then the name is: default)
#user password 0 role superuser access all
#show context | include password (make sure the changes have taken place and are correct. You should see both the new user and the default admin user)
#com wr
Start new SSH session and login as new user. Test to make sure you are allowed to make changes and have access to all functions.
From this new session run the following commands:
#conf t
#management-policy (if you haven't created a new policy then the name is: default)
#no user admin
#commit (
DO NOT write to memory yet. Check your config first)
#show context (make sure the user 'admin' has been removed and only the new one was retained. If the config is correct proceed to next step; however, if it's incorrect, reboot the AP. If prompted to save the config, choose 'No')
#wr mem
Note: When you enter the new users password, it will be a cleartext; however, after saving the config, the password will become encrypted.
Christoph S.