Double click on the "Outlook Express" icon on your desktop.
To edit an account click on "Tools" and then click on "Accounts". This will open the "Internet Accounts" window has shown below. Make sure you click on the mail tab to see your email accounts if any.
Now click on Properties button to show the window below.
In this box change the email address and reply email address to the desired address. Such as firstname.lastname@example.org or Sarah@mummeproducts.com.au etc etc.
Going back to step 1, you can set up several email addresses so whoever
is sending the email can select their own address to personalise email.