To upgrade or downgrade your account, just visit Settings > Account & Billing.
Select the tier that best fits your needs.
If you want to downgrade, you need to make sure that your account can indeed be downgraded. For example, if you're at the ShootQ Studio tier level, but you'd like to downgrade to ShootQ Startup, you need to make sure that you have no more than 3 users, 1 public pricing page and 1 brand. When you click "Downgrade" at the bottom of your tier selection, you will be taken to open a help ticket. A customer care representative will make sure your account is ready for the downgrade and will finalize it for you.