How do I change credit card info in my keychain?

Go to Settings>Safari>Passowrds & AutoFill>Saved Credit Cards, tap the card, enter your passcode, tap Edit, tap the expiration date, change it and tap Done.

Try going to Safari>Preferences>Autofill, click Edit next to Credit Cards, select the card, click Remove, then click Add and add it back with the new information.

Just got the same question today, and hit this page. I don't know if this was possible 2 years ago, when the question was posed, but today it is possible to click on the card, and then click on the detail you want to change (Description, Card holder, Expiry date, whatever) - and it opens edit box.

What about on a Mac, not an iOS device?

Thanks.