To change your default password for your email address, you must login to your online Webmail.
To login, visit mail.yourdomain-name.co.za (replace the 'your-domain-name' with your real domain name).
Use your full email address as username and use your current email password.
Once logged in, on the top right corner there is an option for 'Settings', once that is clicked you will have an option on the left to 'Manage Mailbox'. Login again with your email credentials and your first setting option will be 'Change Password'.