You cannot remove or delete an invoice that you have created. If you have created an invoice that the customer does not have to pay due to faulty information on the invoice, or for any reason you should credit the invoice. To do this, you create a credit invoice as a voucher against the invoice you wish to remove. 

In order to credit an invoice you look up the invoice and select 'Manage and credit'. Then you can choose to credit the entire invoice or just a part of it. A credit invoice is then automatically sent to your customer.