You can have many credit and debit cards saved in your Wallet app to use with Apple Pay, but only one pops up as the default when you double tap your side button to make a payment. Of course you can then choose a different card, but why have that extra step? If you have one card you like to use in almost every case, you should have it as the default option. Here's how to switch your default payment card for Apple Pay on iPhone:
With three quick presses of your Home button, Apple's Wallet app gives you instant access to your credit and debit cards that are linked to Apple Pay. But, like the one in your back pocket, it can hold much more than credit cards. Wallet can also become an organizer of coupons, concert and sporting event tickets, gift cards, airplane tickets, movie passes and more. This keeps everything at your fingertips so you don't need to go searching through emails or fumbling through a purse while a line of impatient movie-goers forms behind you.
Apple Pay is a convenient and very secure payment method that can be used both at brick-and-mortar stores and for online purchases. To use it, you first have to add a credit/debit card to your Wallet app. If you use the service often, chances are you have added several credit cards and at some point will want to remove some of them, either because they have expired, become compromised, or you simply don't want to use a particular card anymore.
If you have an electronic boarding pass for a flight it's nice to have it easily accessible and ready to present, rather than fumbling through emails and loading web pages. If you have added it to Apple Wallet (which you can usually do when you receive it), you can quickly get to it from your Lock Screen by double tapping the Home button. This launches Wallet, and you will see your contents as a bunch of tabs at the bottom. Scroll through them until you find your pass, then tap on it for a preview. Tap it again to bring it up fully.
Apple renamed the Passbook app to Wallet. After updating to iOS 9, the Wallet app will replace Passbook on the iOS home screen.