Navigate to the install file:

Steam: Library Tab > Right Click on "Derail Valley" on the left bar OR Select the "Manage" button on the game page > Manage > Browse Local Files

OR

C:\Program Files (x86)\Steam\steamapps\common\Derail Valley


Navigate to \DerailValley_Data\SaveGameData

Create a folder and move all the old "savegame" files to. Or just delete them.
Move the new "savegame" file to the "SaveGameData" folder

Play the game!