Once a CP is in "Final" status, the CP is locked from editing. The CP can be amended based on 2 scenarios:
1. CP was approved by required parties and status was automatically changed to final copy.
You can simply change back the status by clicking "Cancel Approval".
2. CP status is in the "Final" status without any approval done.
Simply change the status to working copy and save the CP. You would need to refresh the page again after the CP has been saved back to "working copy" status and you would be able to continue with the editing.