If you know your email password:

1. Login to webmail as normal.
2. On the splash page (where you can select your webmail program as Horde Roundcube etc..), select "Change Password" at the bottom.
3. On the next page, fill in the required information and click "Change Password"

If you do not know your email password:

1. Login to cPanel as normal.
2. Locate and click on the "Email Accounts" icon in the "Mail" category.
3. Locate the email account in the list and click the "Change Password" link.
4. Enter your new password and click "Change Password" to continue.