Instructions are right in the tech handbook... just need physical access to the controller via RS232 and a terminal program, it's one line of command. There are literally dozens of tutorials online on how to do it, and if you search this forum I am betting you will find it as well.
Off the top of my head I think it's ResetAllLoginPasswords or ResetLoginPassword "system" to just reset the system user... Just had to do this a couple weeks ago to a 3300 and a MiCollab.
EDIT: Since you labeled this "3300 ICP" I am moving it the correct group, this area is for older SX platform systems, which is a very different process.