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

Below you will find lists of all messages, active messages, inactive messages and drafts. 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 Display 'active' messages and then copy. The copy is now under Display 'drafts' only and can be edited.

Change the mailings by clicking on Replace active 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 Display 'inactive' only. The selected draft moves to Display 'active' only.