how can i update a completed form?

I don't want to integrate with another database like Zoho Creator but I do want to be able to go back to a client form and add notes over time.  Can I do this without moving the data to a new system?

You do can update/edit the existing submission data of a form.

You can do this in the View Submissions page.

Use the EDIT button (located at upper-right corner) to start editing the current submission.

Thanks.