how to install apple pay on my iPhone 5?

You don't. Apple Pay is only available on the iPhones 6 and 6+ (and iPad Air 2) which have an NFC chip. The iPhone 5 does not have the hardware required.

If you want to use Apple Pay with your iPhone 5 you will need to purchase an Apple Watch to enable the feature.

As Meg points out, the iPhone 5 does not have the required Hardware for Apple Pay.

https://www.apple.com/apple-pay/

Apple Pay is compatible with these devices.

I'd forgotten that the Apple Watch would allow Apple Pay on older devices. Good call, Phil.

Thanks. Meg.