Before you can Void an Invoice you must first make sure that:
Any issued stock items on the Order have been returned to Inventory/stock. (This can be down when voiding the Invoice.)
Any Payments on the Order have been Unapplied and/or Voided.
To Find out more on how to Void a payment, look at this Article: How To Void A Payment
Void The Invoice
To void(cancel) the Invoice click on the “Order Status” dropdown next to where it says Invoice at the upper left and select ‘Voided’. Then give a reason for the cancellation and click “Save“.
Voiding an invoice is equivalent to cancelling an order.
If you have items from your own stock assigned to the order then you must return them to a warehouse. You will be prompted to do so when voiding the invoice:
If you want to return the stock to multiple different warehouses you can do so aftwewards on the 'Inventory' page OR you can do so individually for each line item on the order see: