Step 1: Go to Settings, Scroll to Plans and Pricing and Click Manage:
Step 2: Click "+ Add payment method" to Add New Card
Note: If you're updating your current payment information and not adding a brand new card you'll still need to add it as a new payment method first if your Default card is the only card you have on file.
Related questions: How do I manage my payment methods? Where do I update my card on file?