You can't revise an invoice or attach additional receipts once it has been approved by your client.
However, if your invoice has not been approved, you can cancel the invoice and re-issue a new one. To do that, select that pending invoice in your Invoice page, and click "Cancel" on the lower right of the invoice.
On the other hand, if your invoice has been denied by your client, you can revise the denied invoice and resubmit it to your client. You will not lose the original content of the invoice. To resubmit an invoice after it has been declined, you can go back the invoice you have submitted (Invoices page -> Pending tab), edit details of the invoice based on client comment, and click "Re-submit" on the lower right corner.