From the database server, you can do any commands you want as the ‘root’ database user (which shouldn’t have a password, but must be accessed from localhost).
Connect to the database server as ‘root’ and log into the UI for MySQL:
mysql -u root mysql
Once logged in, use a command like:
update user set password = password('thepasswordyouwanttouse') where user = 'freepbxuser';
The password is probably stored in one of the config files (I’m not sure which and my test machine is hosed right now, so I can’t look).