If you're looking to start fresh with your study progress, here's how:
Select the deck or a specific sub-deck you want to reset.
Navigate to its settings and hit "Reset Progress."
Afterward, all your learning progress will be wiped clean, and all cards will be moved back to "Not Studied."
Resetting progress for a sub-deck won't impact other sub-decks within the same deck.
Keep in mind that progress will also reset automatically when you switch between learning algorithms, like from Spaced Repetition to General, and vice versa.