Copy the file to your desktop as a safety measure.

To "unlock" a specific level via skipping, enter the mission you have access to and press Left Ctrl + Left Shift + F12

: Widely considered the hardest mission in the game, featuring six complex objectives in a snowy environment that can even cause graphical lag.

(Invoking related search suggestions...)

: The mission will immediately end in a "Success" state, unlocking the next level in the campaign. 2. Save Game File Method