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It use to be done via your bigpond account but it can now all be done via your Telstra account. Go to the Telstra home page, login to your 'My Account' page, login and then you will see the listing for your bigpond internet. Click on that and the internet account information page will be shown. Select the blue button called options and select Change Plan.
This will actually bring back the old bigpond My Service page where you can select your plan and then go through the process to change your plan (this may chnage as I suspect Telstra are still working on changing pages like this to be streamlines and linked directly with your Telstra account)..