You can change the title on the HTML email by editing your email draft, edit settings, go to HTML Tools, from here you'll see the HTML code of your email, look for the title tag and make your change. Then hit the save button. The only thing is that by doing it this way, you'll have to clean up the text version of your email. Also this will disassociate this email from the main email template that it uses.
You can also change the title from the main email template in design studio.