posBoss doesn't currently support reversing a discount after it has been selected and applied to an order or tab.
However, when a mistake has happened there is a way to add back the value to the order.
Note: Please don't try to void off all the items as you will end up with a negative balance and a look of frustration.
Here's what to do:
Set up an Open Priced button - call it something like "Corrections" or whatever makes sense to you.
You then add it to the order and enter in the price of the discount applied in error eg: enter in 80 for $0.80 or 120 for $1.20.
If you don't want the discount and correction to appear on a receipt for the customer, you could create a new order and pay it off for them, then go to the incorrect order, add the correction and void the other items - leaving the order with $0.00 balance. Press pay and cash to close off and remove that order (won't affect cash up).