Most of the fields are greyed out on a paid invoice or part paid invoice, the only way to edit these details is to remove the payment(s) and return the invoice to the unpaid tab. 

There are two ways to remove a payment, you can de-allocate it and re-allocate after you have made changes, or you can delete the payment and re-create it in future. 

If the invoice has been included in a submitted VAT return you will be unable to delete the invoice so will need to reverse it instead.

Step 1.

Navigate to the Sales > Invoices > All.

Step 2.

Click on the invoice number that you would like to edit.

Step 3.

Click on the Edit button.

Step 4.

There are only certain sections of an invoice which can be amended. These are Project, Reference, Theme, Description and Account fields on a paid invoice.

Once the changes to the invoice are complete click on the Update button.

If you would like to edit the greyed out details of a paid invoice you must first delete the payment(s) on the invoice.