Go to Responders | Articles. There you can create new items for the autoresponder to send, or edit current content and schedule. Edits and schedule changes will only affect subscribers who have yet to reach that part of the autoresponder’s sequence.
IMPORTANT: Once a subscriber has finished the autoresponder they will not receiving any further messages from it, even if you add content to the end of the sequence.
If you want to send a multi-step responder you should plan and craft all the articles up front before taking the responder live.