You change your payment method in your Apple Account, not in the Soco app, because Soco subscriptions are billed through Apple.
Why Apple manages your payment method
Apple manages your payment method because Soco subscriptions are billed through Apple In-App Purchases. The card on file lives in your Apple Account, so Soco can't change it or charge a different card for you.
How to change your payment method
To change your payment method, update the card on your Apple Account from your iPhone:
Open the Settings app on your iPhone
Tap your name at the top
Tap Payment & Shipping
Tap the payment method, then tap Edit
Enter your new card details, then tap Done
This updates the card Apple uses for Soco and the rest of your App Store purchases. Apple's full guide is here: If you need to change or update your Apple Account payment method.