The following steps will guide you to change or edit one email's subject line quickly in Outlook. Please do as follows:

1. In the Mail view, double click to open the email whose subject line you will change.

2. Now the email is opening in the Message window. Please click the button at the bottom-right corner of message header to expand the message header. See screenshot:

Note: In Outlook 2010 or earlier version, the message header is expanded by default, please skip this step directly.

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3. Now the message header is expanded. Please edit the subject line directly, and then click the Save button. See screenshot:

So far, you have changed/edited the subject line of the specified email already. Please close the Message window as you need.

If you need to change multiple emails' subject lines in Outlook, above method will be a little tedious. Here, this method will guide you to change the folder's view setting, and edit emails' subject lines in the Message List directly. Please do as follows:

1. In the Mail view, open the folder containing emails whose subject lines you will change, and click View > View Settings. See screenshot:

2. Now the Advanced View Settings dialog box comes out, please click the Other Settings button. See screenshot:

3. In the popping out Other Settings dialog box, please check the Allow in-cell editing option, and then click the OK buttons successively to close both dialog boxes.

4. Now you return to the folder, please click View > Reading Pane > Off to close the reading pane. See screenshot:

5. Now in the Message List, please click the subject line you want to change, and then edit it directly as below screenshot shown.

And then repeat this step to edit multiple emails' subject lines as you need.