On the My Plan subpage, you can switch between different pricing plans.
You'll find more information about the plans here: https://patchstack.com/pricing.

If you haven't subscribed to any paid Patchstack plan, you will see screen as such:

Entering your billing information

To enter billing information, choose the plan.
After choosing, you will be taken to the form where you can add your credit card information.

Upgrading / downgrading your plan

If you have already subscribed to a Professional or Business plan, you can manage your plans from the same page: