You need to login to at gator.net.au and the go to: Settings --> My Account --> Set Mobile Number
This will take you through the verification process again and after confirming the new number, it will be saved as your number going forward.
If you have a phone connected to your account that you no longer wish to allow access (eg, you've changed phones), you can remove it from the web interface.
Edit App Access
Open the full web app here: https://gator.net.au Then use the "Settings" button and change to the "App" tab:
