How to change your credit card on iPad

if it was me then I would try

If you need to change or update your Apple ID payment method - Apple Support

and maybe

If your payment method is declined in the App Store or iTunes Store - Apple Support

if that did not help me out then I would reach out to their support channels

Get Support (apple.com)

And or

Contact - Official Apple Support

And or

Contact - How to Contact Us - Apple

Goto Settings, tap on your account at the top of the menu which has the name from your AppleID. Then you can tap on “Payment & Shipping” once in there you’ll be able to change all the credit card or payment information that you need to update.

With Apple, if they are going to charge you through the App then it’s going to use the information from Apple as it charges you through Apple and the developers don’t get any information from Apple. If this is the case, then you can see the other post I put that shows how to update the credit card information. If this isn’t the case and you’re getting billed from WeighWatchers directly then you’ll have to get in touch with them to update your information.

I have been trying to update my credit card information on my WeightWatchers app, but it isn’t letting me