So, you're site has grown, you're getting more traffic, or you have more domains. For whatever reason, it now makes sense for you to move up to another plan.
Changing plans as a pretty simple process, all you need to do is log into your control panel, and click the paper and pencil icon next to the words "Plan:" foll but I do what the new cost will be owed by your current plan name in the left hand column.
On the next screen that comes up, you'll be able to select a new plan. When you make the change, the control panel will tell you what the new cost will be and will ask you to verify before proceeding.