If you get any mistake in any of your existing invoice, you can send a credit note to your client. The credit note will reverse the amount of the invoice.

To create the credit note, please go the the invoice details view. Click on the Create credit note menu form the Actions dropdown. 

A confirmation modal will appear there. You can create credit note from there.  

The main invoice's status will be changed to Credited and a new credit note will be created. You can see the credit note on the invoices list view.