Under the tab Archive > Email, you will find all sent mailings. Select your mailing and click "Restore a copy" under Actions on the right side.

Next, you can change the subject of the copied mailing. If your mailing is based on a template, you will be asked whether you want to convert it into text and HTML. If you wish to continue working based on the template, click "OK". Your copied mailing will now appear in your unsent emails under the tab Mailings > Email.