To start, go to your client area which can be found here.
Next, find and click the button that says "Order New Services."
Once on the server selection page, select "Product Addons" from the categories section on the left.
Once on the page, select the addon named "+CPU" and click "Order Now." Voila! You'll be taken to the checkout screen. If your not sure what to do now, read Configuring Your Plan.