In the QuickClick navigator, near the bottom there is a link for "Refund or Balance Owing". Once you click on this, there is a section for "start direct deposit" where you can update your direct deposit information.
One thing to note is that if you wish to make changes to your direct deposit situation, you will NOT be able to netfile your return. You can go to the CRA's website for more information about how to set up direct deposit: http://www.cra-arc.gc.ca/directdeposit/