If you are a Telstra Bigpond Internet customer, you select a plan to begin with, but you can quite easily change this plan by logging onto the Bigpond website and doing it yourself!
Visit www.bigpond.com and click on "My Bigpond" in the top right corner of the site. After logging in, one of the first things you will see is a link called "Change my plan".
Click on this link and follow the instructions to change your plan. Other plans may cost you less or more depending on the speed and MegaByte allowance you select.