Refrence a list from another list

I have a list that automatically gets added to when a user completes a form (lets call this List 1) and another list with all people who are required to complete the form (and List 2). So when the user submits the form, List 1 recieves it. I need List 2 to be updated and change the ‘Form Completed’ column to be yes.

I have tried this already within a SharePoint workflow but I recieve an error.