how to delete an account to start a new game?
As far as I am aware, it is not possible to delete game progress tied to a specific PSN/Gamertag.
Perhaps, contacting support and putting in a request for such an action may result in a desired outcome.
Otherwise, you can always create a new account tied to a different PSN/Gamertag… but, I can see why you wouldn’t want to. You just want to start over.