All changes to your billing can be made from the My Subscriptions window.
You can access this window from the switcher.
The new plan appears with a breakup of the billing amount based on the number of users you need. You can also find the total cost breakup including add-ons on the right. Click Update Subscription.
The updated billing information appears with a success message on the top.
Your upgrade will be immediately reflected and an invoice will be generated.
Note - If you are an offline paying customer (if you do not have a credit card saved in your subscription and have an alternate payment system), you will have to clear the new invoice in the defined timeline as per your chosen payment method.