There are 2-3 ways. If you're using non HM Classic, go to your Global Settings (top right corner). Once there, go to Device Management Settings on the left side. That is the default password that gets applied if no admin password is set on the hardware directly. That uses admin as the username.
To change both (HM NG and HM Classic). Go to the Monitor tab. Select the gear you want to set to the same password (or all of the gear). Hit the edit button and on the left go to Device Credentials. Set the username and password you with to use and make sure the gear gets updated.
If not using HM at all, from the cli the command is: admin root-admin <username> password <password>
