How do I remove the "Received" or "Completed" stamp from an Income&Transfer reminder, if the status was prematurely changed?
To answer your question, there isn't any sort of undo button or undo feature for if you accidentally mark a reminder as received prematurely.
That said, you can delete the transaction from the register and then (if its a recurring reminder), edit the series of reminders. To do this, you can follow these steps from an earlier Community post (Thank you @Boatnmaniac for this solution!):
If the reminder is one time, rather than recurring, then the process would be similar. After deleting the transaction, you would need to go to the Bills & Income tab and recreate the reminder.
I hope this helps!