My wife’s iPhone 6 battery has stopped holding its charge. How much will Apple charge me to replace the battery?

$49. See→iPhone Battery Replacement - Official Apple Support

No, but you may have to wait several hours if you don’t. Go to https://apple.com/retail, search for the store you want to visit, and there will be a link to make a genius bar reservation. If you don’t have one everyone with a reservation will be served first.

Do I have to make an appointment to have a new battery put in my iPhone 6?