You cannot change a campaign marketing message once it has been sent. You can change Everyday marketing at any time.
1. Go to the Admin tab and select the marketing message you want to edit.
2. Change the details as your require and click Save
3. Change the List of who you are sending the marketing message to if necessary and click Save.
4. Make any other changes you require and click Save.
This will update the marketing campaign to take into account your changes.