How to change the "from" name, not the password or email addres?

I have the same problem as "S", because my "from" name in yahoo is different than my "from" name in Outlook Express. Because of this if I send an email from Outlook and the person responding sends back their email by hitting reply the email will not be delivered back to the email address it was sent from, it will be sent to my main (default) email account. Please tell me how I can make this change without completely removing my yahoo account and reenter it.