1) Login to your webmail site, generally located at http://webmail.<yourdomainname>.com where <yourdomainname> is the same domain as your website. 
2) Click Options Tab in upper right
3) Click Login Menu Item on the left
4) Enter current password, and new password twice. Choose a strong password, at least 10 characters, or use our easy Password Generator
5) Click Save at the bottom
6) Click Log Out at the top right