To change your cPanel password, assuming you know the current password, first log in to your cPanel:
http://www.yourdomain.com.au/cpanel
Once logged in, towards the top of the page click on Change Password
The Change Password page will now be shown as below. Enter your Old Password, then your New Password and then your New Password (again). When ready click Change Password.
Please note: this will change your password for cPanel as well as your main FTPaccount.