Many users ask about editing the subject line of messages, usually because they plan to save the make the subject more better reflect the content of the message.
This is possible - and simple - just open the message, select the subject and begin typing. You don't even need to do anything special, beyond opening the message. You can select the subject and type over it or backspace to delete all or part of the subject, or add to the subject.
After editing, click Save to commit the changes, or click close and don't save to revert to the original subject.
You can also edit the subject when using a list view with in-cell editing enabled.
In an Open Message
You can change the subject line of an open message. The subject field doesn't look editable, and appears grayed in older versions of Outlook which usually means you can't make changes, but you can still click in the subject area and start typing. Note that in order to edit the subject in an open message in Outlook 2013 or 2016, you need to expand the header using the caret in lower right corner.
In an Outlook View
You can make the subjects editable in the Outlook message list when in-cell editing is enabled.
Note: In Outlook 2003 and up, in-cell editing works only when the message list is using a single line view. When you use Conversation view in Outlook 2010 or 2013, you'll need to have more than one message in the conversation to use in-cell editing. If there is only one message, turn off view by Conversation.
In Outlook 2010 or Outlook 2013, look for the option in View ribbon > View Settings or click on the row of field names and choose Customize > View Settings. Click Other settings button to open the dialog shown in the screenshot.
You'll see an option for Allow in-cell editing. Check the checkbox, then click OK, and you'll be able to edit the subject of any item in the folder, without the need to open the item first.
In Outlook 2003 and Outlook 2007, go to View > Current View > Customize view > Other settings button or click on the row of field names and choose Customize > Other settings.
If you are still using Outlook 2002, 2000 or 98, go to Choose View > Current View > Customize Current View > Other Settings button.