Create a backup of your Deltarune Data.win, and save files.

(Just in case)

Instructions, 
Open Steam
Navigate to the Deltarune page on your Library,
On the right should be a Cogwheel, click that.
On the drop down menu, hover the mouse over "Manage"
Then click on "Browse Local Files"
(DO NOT DELETE ANYTHING IN THIS FOLDER, If you do, find it in the Recycle Bin and restore it.)

Copy the file in the PinkBombCrosses folder called "data.win" into the DELTARUNE folder, it should prompt if you'd like to replace the already existing "data.win" click yes (Don't worry, you can always reinstall the original by reinstalling the game)
You do not have to copy this file. or the folder.

Open the game, it should be installed now.