All changes to your billing can be made from the My Subscriptions window.

You can access this window from the switcher.

If you have a single subscription, you will directly see the Subscription Summary. If you have more than one, you will have to choose the Subscription you wish to manage as shown below. 

Select Edit next to the Plan and User section on the screen to change your plan.

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.