This is done on the accounting menu.
Accounting -> Manage Claims -> Individual Activities -> Locate the claim on the grid -> There is a button here that says 'Correct note and set to in progress'.
Clicking that will remove the activity from billing and sends it back to the clinician to make corrections. Once the corrections have been made, the clinician can click the green 'corrected' button on the activity grid which will send it back to billing.