There will be an easy way to delete your save file in the next demo, but for now you'll have to do it manually by going to AppData/Local and deleting the BE_Witches_20_08_2020 folder. You could try using windows key + R, type %appdata% and confirming to access the appdata folder, but if that doesn't work you should be able to easily look for more info on that online.
A new demo is coming out soon-ish, I've been posting public devlogs on Patreon regularly. The end of this one in particular might interest you: https://www.patreon.com/posts/79403516
