Adjusting an Invoice is an easy process. After you find your invoice click view. On the bottom right hand side there are Job Total and Job Total Due clickable amounts. When you click on one of them it will take you to Adjust Job Charges window. Bottom right hand side in green there is a Edit Rates tab. When pressed it will allow you to do changes to the invoice. Make sure to save all changes.
You can also change the pricing on the job that was invoiced, and the updated pricing on the job will fuel changes to the invoice.