Hi,
It is not currently possible to edit and submit the already-filed CT600 to Companies House. You should create another filing for the same company and accounting period, and this time select only Companies House (Annual Accounts) in the wizard. Your financial data from the previous filing won't carry over automatically, but you can re-enter (or copy) the same Profit & Loss and Balance Sheet figures you used for your CT600.
Here's how to proceed:
Go to your Companies page and select the relevant company
Click Create Filing and choose the same accounting period
In the wizard, select Companies House only
Complete the Profit & Loss and Balance Sheet steps using the same figures from your CT600 filing (you can reference your previous filing's details for this)
Enter your Companies House presenter credentials and submit
If you have any trouble or need further assistance, you are welcome to create a ticket on your dashboard.