Note: Use a text editor such as notepad to edit the "preferences.dat" file in the "config" folder. Locate the following line:

const bool& ALLOW_LEVEL_SKIP =Bool(); // is FULLY_PROPER_BUILD no yes

and remove the "FULLY_PROPER_BUILD no" part so it reads:

const bool& ALLOW_LEVEL_SKIP =Bool(); // is yes

Press [F8] during game play to advance to the next level.