From the Dashboard of the app, go to Pricing plan -> click on Upgrade now button to upgrade your plan.
From the Dashboard of the app, go to Settings -> General -> App plan -> click on Downgrade link to downgrade your plan.
When you downgrade your plan to a lower plan, you have to delete the pages/templates and you will be limited to the plan that you will downgrade to.
For example you are in a Pro plan and you have 10 standard pages and 1 product template, when you downgrade your plan to Free, you have to delete 7 standard pages and 1 product template, only keep 3 standard pages to use and editing. Because you can only create up to 3 standard pages with Free plan.