The Campaign Title that displays on the Campaigns page, and the Subject Line of an individual mailing are two different things. However, Email Marketing will use the Campaign Title as the default Subject Line, the first time you send a campaign. And any subsequent send will default to the Subject Line used on the previous mailing. But you can edit the Subject Line for each mailing, and the Campaign Title at any time -- completely exclusive of each other.
Edit the Campaign Title of any campaign in your account by clicking on the Edit button under the campaign thumbnail, on the Campaigns page. Then click into the field at the top of the Composer, and edit the Campaign Title, here. Be sure to click the Save button, in order to commit any edits you make.
Edit the Subject Line at the time of sending your mailing. The Subject Line field displays on the second page of the mailing process, and is fully editable, each time you send a mailing. Simply make your changes before clicking Send, and the mailing will have your new Subject Line.