The default password is 12345, but if you have chosen an own password, you need to perform a "Factory Reset and Firmware Revert", which means that the rack mixer will loose all settings. You will also need to update firmware to the lastest version. More information can be found in the Knowledge Base:
https://support.presonus.com/hc/en-us/articles/115004856203-StudioLive-Series-III-Rack-Factory-Reset-and-Firmware-Revert
or in Answers: https://answers.presonus.com/21046/studiolive-series-rack-mixer-factory-reset-firmware-revert?show=21046#q21046