How do I change my auto-pay credit card on line

Log in to https://tv.apple.com or https://music.apple.com and use the Settings section there to update the payment methods for your account.

Link-> Change your Apple ID payment method if you don't have an Apple device - Apple Support

Hello Doublebogey2, If you are wanting to change your Default Payment Card then go to Settings > Wallet & Apple Pay > Transaction Defaults > Default Card > Place a Check Mark on the Card you want as Default for Payment. You can only have one Default.