-- DELETING YOUR SAVE FILE --

First thing you wanna do is disable Steam cloud sync for the game. You do that by rightclicking the game in your Steam library > Properties > Updates, and then uncheck "Enable Steam Cloud Synchronization". Alternatively, you can as well put Steam into offline mode

Next you have to remove the file which saves your progress, which is C:\program files\Steam\userdata[whatever your Steam account ID is]\212480\remote\ast_save.bin. It's recommended to not delete it, but instead backing it up somewhere, because apparently it can happen that Steam synchronization fails, and in this case you can restore it (see below)

This will allow you to play the game on either a new save file or completely without saving at all. The game asks you at the beginning if you want to create a new savefile. It is recommended to play without savefile, since it is faster and makes resetting the game quicker

-- RESTORING YOUR SAVE FILE --

For getting back an old save file from before resetting the game (in case you have a casual / online / 100% save file), there's 2 possibilities:

First one being enabling Steam Cloud Synchronization again (or put Steam back to online mode). When you now launch the game, Steam will ask you in which direction you wanna synchronize. If you choose "download to this machine", the save file that is backupped in Steam cloud will be downloaded and you will have your previous file back.

An alternative way, and also a backup way in case you accidently overwrite the file in your Steam cloud or Steam synchronization fails you (which apparently can happen so be warned), is to manually backup ast_save.bin somewhere before deleting it when you reset the game. However, if the game did not save a save file last time you've played it (for example when you've done speedruns without saving), it won't recognize the file when you move your backupped file back to it's original folder and ask you to create a new save file. But if the game already has a save it's running with, you can just replace it. So open the game, create a new save file, and then replace the new ast_save.bin with your backupped file