On the Email Summary page, use From Address drop down (see item 1 in the image below) to modify the From Address.  This is the address that your email will appear to have been sent from.  Be very careful when modifying the Reply To Email, however, because this is the email account that will actually receive the email if the recipient replies to your email.  If you have questions or would like to use an email address that does not appear in the Reply To Email field, please contact your system administrator.