There is unfortunately no way to switch back to a 'Simple' Deck again.
The reason is that switching to the Advanced Editor adds HTML formatting to cards (so you can add bold, underline, etc.), whereas Simple Decks are just plain text. If we were allow users to switch back & forth, then all your flashcard formatting would get jacked up with code jibberish.
If you'd like to start over as a Simple Deck, you can export your Decks on the website (using the option found in the gear icon) and then re-import them into a NEW class and deck to begin in the simple editor again.