Hello Jo,
Log into the controller via WebUI and go to Configuration >> Management >> Management Policy >> Select the Policy >> Edit >> Administrators >> Select Admin user >> Edit >> change password >> Ok >> Exit >> Commit and Save.
From CLI: https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-change-the-username-and-password-for-WiNG-products-via-CLI
Thank you,
Chris
Christoph S.