Log into the box using SSH (same as when you set up the box). Then run:
sudo /root/mailinabox/management/cli.py user password your@address.com {type new password here}
Replacing your@address.com with the email address that you created when you created the box (or whatever your admin account email address is).
If your not sure what your admin email is then run this:
sudo /root/mailinabox/management/cli.py user admins
Also you can run
sudo /root/mailinabox/management/cli.py
for a full list of commands.
as found here: Reset Admin Password script was moved so I just updated this…