If you're on a plan higher than "Pay as you go," you have the option to purchase additional Cloud Storage capacity. Simply navigate to the Usage page in your account settings and click Manage Cloud Storage.
This will increase your Cloud Storage capacity for an additional fee, which will apply until you decide to downgrade.
For Managed Media, starting from the Pro plan and higher, you have the option to store a larger number of proxies on your workspaces for an additional fee. Once the hard limit is reached, the only way to free up space is by removing files or upgrading to a higher plan.