Go to the My Subscription window. You can access Subscriptions 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. 
You can purchase add-ons by clicking on the Edit/ Buy add-ons from the My Subscriptions screen. 
You can find that a wide range of add-ons that are over and above your plan are available. Typically, they have descriptions on their use cases, and are associated with quantity/monetary value if applicable.
Choose the add-ons that you require.
On the right-hand side, you will notice the update in your pricing break-up. Once done, click on Update Subscription. Click Confirm.
You will receive a success message and the new add-ons will be reflected.
Note - If you are an online paying customer (with a saved card on file), you will be charged based on the add-ons you have purchased, immediately. If you use alternate payment methods (offline paying customer), you may notice limitations in the range of add-ons that you have. Please reach out to [email protected] for support.