Choose your auto campaign in Reporting and go to the Details tab:

At the bottom, you will find a list with all drafts, the active as well as the inactive messages. Create a new message with the button on the right. You can exchange the currently active message with the newly created one.

To copy an active message, click on Active and then copy. The copy is now under Drafts and can be edited.

Change the mailings by clicking on Replace message

and then select the new message out of the created drafts.

The message, which was activated until now, will be moved to the list Inactive. The selected draft moves to Active.