Office 365 (Outlook 2013) - Changing the Subject of a Message
Instructions for editing the subject line of a message to better reflect the content of the message. This can be done in the Outlook Client but not in the Outlook Web Application.
1. Double-click to open the message.
2. Select the subject and begin typing. You can select the subject and type over it, backspace to delete all or part of it, or add to the subject.
3. After editing, click Save to commit the changes, or close out of the message to revert back to the original subject.