If the admin password is not known then it must be reset via the Keycloak admin console. The following knowledge article explains how to enable and access the Keycloak admin console:
Signals Image Artist: Accessing the Keycloak Admin Console
Once enabled, log in to the Keycloak admin console and select Users from the menu on the left-hand side. Now search for the 'admin' user.
Click on the 'admin' users ID (or select Edit from the Actions column), then select the Credentials tab. The password can then be reset as required.