You can edit the autoresponse campaign associated with a lead capture page by going to My Marketing Pages > Lead Capture. Locate the lead capture page you'd like to edit the autoresponse campaign for in the list. In the box to the right of the capture page, locate the heading that says "Message Campaign". You can click "Edit" underneath this heading and select an autoresponse email campaign from the drop-down list. Make sure to hit "save" when you're done to save any changes you make. 

If you're a Plus or Pro member, you can view your Message Campaigns, in addition to edit or create new ones, by going to Email, Text, Voice > Message Campaigns. Just click on the pencil and paper icon next to any message campaign to view the email series inside. Click this same icon next to any of the emails inside a campaign to view or edit the email. 

You can create new email campaigns in the middle section of this page (as shown in the screenshot below).