Your plan comes with a certain amount of space and bandwidth. We've arranged the plans so that it will eventually become more advantageous to upgrade to the next plan level. But if you just need to add a little more space or little more bandwidth in a make more sense to just add more space or bandwidth to your plan.
To add more space and bandwidth log into your control panel. Once you've logged in you'll notice the statistics panel in the left hand column. In this panel you'll see two meters. The first will show you the lot of traffic you've used compared to the amount of traffic your allocated in your plan. The second will show you the amount of space you've used compared to the Meta space you have allocated in your plan. Below each of these meters is a paper and pencil icon. Click the paper or pencil icon to adjust your space and bandwidth.
The control panel will tell you what your additional charge will be per month for the additional space you've added.