This tutorial describes how to change the default email address within Outlook Express:

• Open Outlook Express.
• Go to Tools - Accounts

• Select the Mail tab, this will display all the email accounts that you have setup in Outlook Express. Select the email account which you wish to set as default by selecting with the left mouse button.
  

• With the desired email account selected, press the “set as default” button on the right hand side.
• It is possible to determine which is the default mail account by examining the “type” column. It will show “mail" (default) to the right of the email account that is default.