Any time you make a change to an invoice Gazelle will automatically push the changes to QuickBooks the next time you sync.
You can trigger an invoice to be re-synced by editing:
Customer information (name, address, etc...) tied to synced invoices.
Alternate Billing Changes (when assigning an invoice to a different customer)
Payments recorded in Gazelle
Adding line items to an invoice.
Changing the price of any line item on the invoice.
Changing the taxes associated with an invoice.
Deleting, canceling, or archiving an invoice in Gazelle will not change the invoice in QuickBooks.