Head to ImagineProducts.com and log in. Once you are logged in navigate to Account >View Licenses and choose the license you wish to purchase an update plan for. 

Select the button marked Update underneath the Plan Expiration header. Additionally, you can select the Pencil icon at the far right and then scroll down.

Select either the Annual Subscription plan or the Single Purchase plan, learn more about those here. Input your credit card information and then select place order.