Navigate to the Sales > Sales List on the sidebar. This will present you with all the sales made arranged in a descending order. Find the first column (Action column) in the table then click the Action button. If your display screen is small, then you will have to expand the row details by clicking the blue plus (+) icon to open the action column as shown below. Click Void Sale link and confirm to void the sale.
Voiding a sale is like cancellation of a whole order. The order/sale is canceled with the items restocked. All voided sales are pushed to Products > Voided Sales section.
Click Void Sale