If you have already sent out the email before and it is now in Sent status, you will need to make a copy of that draft so that it is then in draft status. Please see these instructions on copying a sent draft.
Hope that answers your question,
Once an email is sent you cannot edit the sent email but you can make a copy of the email and resend it with the changes. You have to copy your email campaign into a draft format so that you can make your changes and resend the email.
To copy an email click on ‘More” next to the email campaign you want to edit and then select 'Copy' from the drop down
If you are using our 3GE once you copy the email it will ask you to re-name it. Give it an appropriate name and select save. If you are using the 2GE you will need to go to the left top corner and edit the name under where it says ‘Customize Your Campaign’.
Once the email is copied and renamed, make the edits you need to the email itself. Make sure to change the subject line so that your readers know you have made a change to the email and schedule the email to go out.
This FAQ goes into more detail: Copy from the Campaign List View.
I hope that helps you!