Yes, you can downgrade your web app plan at any time.
When you remove seats or downgrade your plan, you do not receive a refund back to your credit card. Instead, any excess payment amount will be credited to your PredictHQ Organisation's account. The credit balance will be put towards your next bill. If you have any credit, this will be shown in your Billing page.
If you wish to downgrade your plan, you need to be an Admin for your Organisation.
If you're not an Admin for your Organisation, get in touch with someone in your team who is so they can downgrade your Organisation's plan.