I would suggest to either join a domain or create as a Standalone CIFS server  I dont think the use case of domain CIFS server that isnt joined is really supported  You could try to change the password from the change password screen of a Windows box Ctrl-Alt-Del Change a Password Instead of the default domain/username enter the CIFS servers and Administrator account. You will need the old password that you defined when you created the CIFS server though
Unable to change the local administrator password on a newly created CIFS server. The CIFS server is not standalone. CIFS server has not been join to the domain. We know the procedure to change the local administrator password. Getting "System Error 64" when trying to perform a "net use \\CIFS Hostname /user:administrator"   What is preventing us from changing the password?
Unable to Change the CIFS local administrator password