Click on the 'My Account' tab in the top navigation.
Under 'Account Details', click on the link that reads 'My Subscription & payments'.
At the top of the 'overview' tab is a summary of the current plan. Inside this box, click the 'cancel' link. This cancels both your subscription and the automatic payments. Ignore the 'cancel' button(s) in green below this box. They are simple mailto: links that are used only if the link doesn't work.
Once you see the 'subscription result: Cancelled!' screen, click 'help->My Subscription & payments' again.
A new green box appears at the bottom of the screen with an 'upgrade/renew' button. Click this and select the plan you want to upgrade/downgrade to.
You'll then be prompted to accept terms and conditions and then enter payment details again (for security reasons we don't keep these on file)