To Add/Remove an Automation to a contact go to your Active Campaign Action. From there, click the green "Add Another Event" button. (This button only appears once a list has been selected and you have saved the Add/Update Contact Event). Then select "Add/Remove Automation".
From there, click into the box and select the Automation(s) that you want to add or remove.
Once you have all of your Automations selected, click the "Save" button at the bottom of the event.