How to do this depends on the type of hosting you have. If you have the older web hosting, you can switch by clicking options on the plan from your my account page and going to the customize tab.
If you have cPanel hosting, you need to call support. They'll have to cancel and refund your current plan and help you get the correct one. You'll need to backup your content before you do this, because you'll have to start fresh in the new hosting plan.