You can delete sales in DataLog by highlighting the desired sale(s), and clicking on the delete sale button above the sales grid.  You can also use the right-click menu on the sale to delete sale(s).

Once you click delete sales, you will see a confirmation dialog box asking if you want to delete the selected sale.  If you are deleting multiple sales, the confirmation dialog box will show how many sales will be deleted.  Click yes to delete them, or choose no to cancel.