How do I upgrade my credit card on my iTunes acct (I can't seem to get my Pandora One back!!!)?

I know I have to do this on my computer, but I don't know where to go to do it!?

In iTunes 12..2 on computer it's under Account on the menu... View Account... and then Payment info on the screen... edit...

You can do it from your iPhone, if you'd like. Go to Settings - iTunes & App Store, and then select your Apple ID. After that, select "View Apple ID". Enter your passcode and then select to view your payment info. Make the necessary changes.