You can cancel your automatic top-up through Easy top-up page. You can continue using your Youth plan with manual top-up. Keep in mind that the manual top-up doesn't come with more data. You'll get 5GB instead of 10GB or 20GB instead of 35GB based on your top-up amount.
If you wish to opt-out from the Youth plan, send SMS 'STOPYU2020'. Any remaining benefits will be removed upon cancelling your plan.