You are allowed very limited editing of finalized invoices in CORE. You can add or edit the invoice memos, change the invoice number and date, and edit the invoice description, provided you have the right security permissions. Check CORE Help Center for details. However, you cannot change or delete the invoice hours, taxes, and other information. You have to delete or void the invoice and recreate it with the changes. Also, an invoice becomes un-editable after it is posted and sent to the client.
Note: You can edit drat invoices using the Invoice Editor.