To cancel an invoice, find the relevant invoice in the records list, select it, and click the Cancel button at the top. The invoice will be struck through and will remain in the document register, but without any further impact on accounting.
A cancelled invoice can only be deleted from the system if no other records are linked to it. If you want to delete the invoice from the system, you must first remove all documents linked to its line items, such as issued delivery notes. You can view the links associated with a given invoice using the Links button located in the upper right corner of the invoice detail.