There are two ways you can change your trading password and both require you to be logged in to your CommSec Account.
You can update your trading password by navigating to Settings > Service Centre.
Alternatively, if you are in the middle placing an order and have forgotten your trading password, you can simply reset it but clicking the 'Forgotten your trading password?' link in the trading login box, entering the mobile number you have registered to your CommSec account and following the prompts to reset your password.