Users must have permission to void an invoice.
For invoices that were paid by credit card, you can only void the invoice on the day it was created. Otherwise, you must issue a return.
Voiding an invoice prompts the user for an explanation.
Review the invoice, either by clicking on the invoice number from the customer's history, or by going to POS > Manage Invoice and selecting the invoice there.
Once in Invoice Review, go to Action > Void.
Best practice is to only void if a true invoice creation mistake was made. If the customer is returning the item, you should create a return invoice.