Once you are logged in and have navigated to the View Licenses page (need help getting there?) choose the license you wish to purchase an update plan for and select the edit button.
Scroll to the section Manage Software Updates and select either the Annual plan or the Single Purchase plan, learn more about those here. Input your credit card information and then select place order.
Learn More:
What if my update plan expired?
Do I have to buy an update plan?