Office 365 (Outlook 2016, 2019, Web) - Edit the Subject Line of a Message
You can change the subject of any message you receive.
1. Double-click to open the message. You can't change the message subject from the Reading Pane.
2. Select the subject and begin typing. You can select the entire subject and type over it, backspace to delete all or part of it, or add to the subject.
3. After editing, click Save in the top left corner of the message window and then close out the message.
4. Your new subject will show in the Reading Pane.
For the most current information, please see the following Microsoft help article: https://support.office.com/en-us/article/edit-an-email-subject-line-08d60328-2a27-473e-8227-0b42a1ff2ce6