If you upgraded your account through MeetMindful.com (not the iOS App), you can change the credit card you have on file for your MeetMindful subscription by visiting the Membership page and selecting change credit card at the bottom of the page.
If you upgraded your membership within the iOS app, your subscription is managed entirely between you and Apple using iTunes and your Apple ID. We are unable to adjust your subscription or your payment method for iOS subscriptions, but here is where to find those settings on your iPhone or iPad:
Go to Settings > iTunes & App Store.
Tap your Apple ID at the top of the screen.
Tap View Apple ID. You might need to sign in or use Touch ID.
Tap the MeetMindful Subscription.
Use the options to manage your subscription.