Go to the Transaction tab and double-click on a transaction. On the bottom right you will see the Void Transaction button.
You cannot void a transaction from the Back Office or after the End of Day Process has been completed. Notice the Void Transaction button is gone in these scenarios:
The item will be returned to inventory in the status it was in so the item can be sold again to someone else. If the item was a consignor's item the consignor's account entry, where credited for the sale, will also be backed out so not to double pay the consignor when the item sell again.
Note that if the item was created on the fly at checkout (items 3 and 4 under add item to a transaction) it will not be added back into inventory. Stores that just enter inventory at checkout are not usually concerned with what consignor's keep in their booth so the decision was made to just erase the item so it can be entered again or be returned / donated by the consignor so not to remain as inventory in the store.