To delete your save on Windows, go to “%appdata%/Godot/app_userdata/History Beast of Burden”, and delete “progress.sv”.
Deleting your save on the web build involves deleting cookies related to the game. I’m not sure on how exactly to do this without potentially wiping saves for other itch.io games, so I recommend using an incognito/private browsing window instead. That way you can start from scratch without wiping your existing save.