You can unmark topics via the Study Something Specific tab. For material that you've learned in the past, which is no longer pressing for your immediate study needs, you should mark these topics as "Past." You should aim to mark all previously learned topics as "Past" for best practice. These topics will remain in your overall recommended review schedule, but will be de-prioritized below any material you've marked as "Current" and/or "Urgent."
If you hit the "Manage Marks" button, then you can change or even unmark topics in bulk.