To void an invoice, go to the invoice in question, hit the action button (three dots to the right of the invoice), and select Void Invoice:

Important tax considerations

If you use an accounting integration with Fergus, voiding the invoice in Fergus will also void it in your accounting platform, once an invoice has been voided it can't be unvoided. 

It will also not create and assign a credit note to the invoice within your accounting platform.

You should manage the invoice in your accounting package with an opposing credit note, in order for the applicable tax to be accurately represented in your Accounting System.

Simply voiding an invoice in your accounting system, does not follow any specific workflow this process, so you will need to use your accounting software to manage this.