You can easily edit the subject line of any campaign you are working on. From the "Review your campaign" page, you'll see a "Subject Line" section under "Campaign Summary." This section displays the current subject line, if one exists. To the right, there is an "Edit" button. Clicking that button will take you to the "Enter your subject line" page where you can change the subject line. Then, just click the "Next" buttons to return to the "Review your campaign" page.