Contact your ISP for your new account settings and use those to configure
Outlook with that account.
Within Outlook, you can do this via Tools-> Account Settings...
Just go into the e-mail accounts that you defined in Outlook and change
the value of E-mail field. Of course, if you also switched to a
different e-mail provider then you'll also have to change the mail hosts
specified in those e-mail accounts defined in Outlook.
I assume you have an account attached to Outlook already. If it is a
standard internet address from Hotmail or Gmail or the hundreds of other
services, you need to set up an Account in Outlook. This is under Tools ->
Account Settings.