Once the campaign is been published, you can ONLY edit the TEXT/MESSAGES and the TIMING DELAY.
You CANNOT edit the ACTIONS, ORDER OF THE SEQUENCE OR ADD OR REMOVE ANY SEQUENCE for any campaign that it is running and already published.
HOW TO EDIT:
From the specific campaign, click the three dots located at the right side of your campaign dashboard then select Edit Sequence.
You can now edit any message or timing from that campaign.
Take note that if you decide to change any message or delay timing, it will only affect your leads that haven't gone through that step and moving forward.