While you can replay levels, the game will always spit you back out at your furthest completed level. Which means that if you try to start a run at level 1 after completing the game, once you finish level 1 you'll be sent to level 20.

To combat this, I've written a script that will reset your level progression to 0. If you would rather not use the script, you can edit the registry value yourself.

Look for the HKEY_CURRENT_USER\Software\unitedvirtualassets\Santa is Real registry folder (using windows registry editor), find the key named achieved_level (it will have some extra numbers/text after it) and set it to a decimal value of 0.