Decide which paid subscription plan you seek to upgrade to.
See pricing page for options.
For the Professional/monthly plan, your account architects can sign up for this plan via the Billing page - once valid billing/card details entered. To access your Billing page click on "Account settings" in the profile image dropdown menu, top right of the main navigation bar and then "Billing".
For all other (discounted) subscription plans, please contact us to activate your new subscription.