Studies automatically updates the due status of all of your notes once a day, but you can also force the status to update yourself. This might be useful, for example, if you are using cramming mode, and want to refresh the due notes multiple times in one day.

To refresh the due status of notes in a stack, select the stack in the Library, and tap the action button (bottom-right on iPhone; 2nd-from-right on iPad). Choose the Refresh Due item.

If you tap the action button in the main Library list, you can choose Refresh All Due to update the due status of the whole Library.