You can change the email address used to log in to Jamf Now by visiting the Account section of Jamf Now. The Profile section of your account will give you the option to change your email address.
You will need to type your account password and the new email address you wish to use with Jamf Now. Look for a confirmation email at this new address to complete the process. Once you verify the email change, you will be able to log in to Jamf Now using this new email address.