Login to the online webmail/macmail service using your current password.
In the top right of your online webmail/macmail service, select Options.
Select 'See All Options'. You may alternatively choose to select 'Change Your Password' directly from this dropdown list and skip to Step 5.
Select 'Change your password' from the list on the right.
Fill in your current password and create a new password.
Be sure to save your new password.