Invoices cannot be modified after they are posted. To modify an invoice made in mistake, you need to reverse the invoice and create a new one to replace it. You do have the ability to adjust an posted invoice though.
To adjust unpaid invoices, you would do what is called an invoice adjustment.
To refund/return on a paid invoice, follow this .
Make sure you have the different type of invoice adjustments defined. Invoice adjustment type is defined at Enterprise Setup -> Invoice Adjustment Types. Some typical examples of Adjustment Types would be "Interest Charged" or "Early Payment Discount".
Go to the invoice you want to adjust, you will see a button called Adjust Invoice at the bottom.
After clicking on it, it will ask you to pick an adjustment type, GL Account to Record Adjustment In (which currently only allows expense accounts.). Typically you would categorized any write down on invoices as expenses to the business.
Enter the amount and then adjustment date. Positive amounts will increase the total value of the invoice, while negative amounts will reduce the value of the invoice.
After saving this adjustment, this effectively change the amount owing on the invoice by the adjustment amount. The resulting GL transactions is to credit accounts receivable and debit the account chosen for "GL Account to Record Adjustment In:”.