Nrgymover,
To change the password on a phone attached to a Mitel 5000 it is under the extensions programming.
System > Devices and Feature Codes > Phones > {extension} > password
Typically it is the same as the original programmed extension number. That is if the extension was originally created as 1000 then changed to 1234 then the password would still be 1000.
Thanks,
TE