Best practice for managing an invoice that needs to be changed is to void the original invoice and recreate the invoice. While it is not best practice,  you do have the ability to make changes to the line items on an invoice.

You will not be able to change the invoice if:

• The invoice has been paid. In this case you would need to unapply the payment before changes can be made. See Unapply A Payment and Apply an Overpayment for further instructions. 
• You are outside the Allow Deletion for (days) configured in your Finance Settings. See Configure Default Finance Settings for further details of this setting. 

1. On the contact's Billing tab, click into the invoice number
2. Make the desired changes. You will not be able to change the existing line items or the amount of those line items. You will need to remove the line item and then add new line items.

   

3. Click Done.