To start, go to Manager > Sales.
Single-click the sale you wish to void, then click Void at the bottom of the screen.
Once prompted, type in the reason you are voiding and click OK
The sale is now void. The original sale will turn gray, and a new transaction will be added with a negative amount of the original sale.
You can view the void reason that was entered by double-clicking on the Description field.
Backdating the Sale
If you need to change the date/time of either transaction, check out this guide to find out how.