There are times when invoices are mistakingly sync'd to QuickBooks or Xero. As you now understand there is not a feature from within Kickserv to edit an invoice that has been sync'd. You can edit the invoice from within the accounting software.
How to remove an invoice and job from Kickserv
Locate job in Kickserv
click Delete Job
Now return to your accounting application
and make the corrections before you run sync again or it will undo what you just removed from Kickserv.
Return to the Kickserv job
Click the dropdown next to the edit job
Duplicate job and assign to same customer
Go back to the original Kickserv job and remove it
Navigate to your accounting software and remove the invoice