Is it possible to edit an existing audience list under the Contacts tab?

Question

Yes, you can upload CSV files into existing lists to add new contacts to the list by following these steps:

Navigate to the Contacts tab and click on Add Contacts.
Once you've chosen the CSV file you want to add to your contact list, click Next.
From here you can decide whether to create a new list, add to an existing list, or add to your unsubscribe list by clicking on the Select an audience drop-down.