Paste this into your Windows File Explorer address bar:
%USERPROFILE%\Documents\Fernbus Simulator\SaveGames
You will see folders named SaveGame0, SaveGame1, etc.
Without this step, Steam will restore your old save.
If you have found a save file that claims "100 work," follow these steps to install it correctly. This process replaces your current progress, so make a backup of your existing saves first.