On the Sidebar > Storage, is where you can upgrade your Storage Subscription Plan by pressing the “My Subscription” button.
You will get a range of subscription options, it is up to you to choose which one suits you best.
After deciding all that’s left to do is hit the “Subscribe” button for the chosen plan and proceed.
Even If you're on the Free 300MB plan, or upgraded it to one of the available Subscription plans, you will be able to see the used and left amount of storage, and how much storage is taken by pictures, documents, and other files under Sidebar > Storage.