You may be out of luck. You can default the database and use the default password, but you'll lose EVERYTHING, even, I believe, the license. Which makes the PBX an attractive paper weight.
There's also a super-secret password that Mitel support can use to get into it but you must be a licensed dealer and have control of the account...if you're the end user you need to get a dealer involved, make them your dealer-of-record, and get them to change the password. Mitel charges for this, by the way. One more chance: if you know who installed the old system your new dealer MIGHT know the old dealer's usual password. Or, since you've moved out of the old dealer's territory you might be able to pay the old dealer to either give you the password or reset it for you using join.me or something.