CORE allows editing of draft invoices using the Invoice Editor feature (invoice details > Edit Invoice screen). You can make changes to the draft invoices and then finalize them. Check CORE Help Center for details. 

Typically, you should edit the time entries before or during the creation of the invoice, e.g., include or exclude entries, apply Write-Up/Down, etc. You cannot change the time entries on an invoice after you create it as the invoice amount is fixed for consistent accounting (whether draft or final invoices). All you can do is add more entries to it later using Progress Billing. However, that still does not change the invoice amount, but re-adjusts the value of entries. Check CORE Help Center for details. If you want to edit the entries, you should delete the invoice and recreate it using the entries you want.

Note: CORE allows limited editing of finalized invoices.