To do so, follow these steps outlined below:
From the Volusion Log In page, log in to your store.
At the bottom of the navigation menu, on the left hand side of the screen, you'll see the customer name you entered when creating your store. Click your name to view the "My Account" screen.
Click the Billing tab.
Locate the Account Overview section.
Click Change Your Plan.
Make the appropriate selections by choosing a new plan and clicking the Let's Go button.
You new plan will use your current billing method. To change your billing method, check out this article.