To view, upgrade or downgrade your subscription plan, go to Store > Settings > Billing
Depending on your plan you can choose to updgrade or downgrade your plan, under the plan details.
Note: This only applies to accounts which are on an automated monthly subscription plan for standard offerings. If you are an enterprise customer or have choosen a custom plan, please talk to your account manager for any updates.
To view your past invoices, go to Store > Settings > Invoices