Once a task is marked as "Finished" in the Shop Floor Control app, an "Undo" option is displayed for 5 seconds to revert the action.
If you missed the "Undo" button, then you cannot revert any done tasks directly in the SFC app.
Please contact the owner or a user of the Katana MRP main app. A task can be reverted in the Katana MRP app by navigating to the related MO and changing the operation status on the MO.