Each site is on its own plan. You can move a site between the Starter, Business and Pro plans as its needs change, for example when it starts getting more traffic or needs Redis caching.
Change a site's plan
Open the site and click Change Plan at the top of the page, or use Change Plan in the Quick actions panel.
Review the plans. Each shows its price, included storage and bandwidth, and features.
Select the plan you want and confirm.
The change takes effect immediately. Billing is post-paid, so you are charged for what you use on the new plan.