sudo maas changepassword <user>, where user is the user you want the password changed:
cerdea@capiau:/var/log/maas$ sudo maas changepassword maas-root
Changing password for user 'maas-root'
Password changed successfully for user 'maas-root'
Of course, you may need to find which user to change the password for:
matsubara@tancho [~]$ sudo maas shell
In : from maasserver.models import User
In : User.objects.all()
Out: [<User: maas-init-node>, <User: admin>]