How much does it cost to replace the battery in an iPhone 6? Do I need an appointment?

I recommend scheduling an appointment. Not necessary but if you don't you may end up waiting a long time. With an appointment your wait time is dramatically reduced.

I don't know how much but you need to take an appointment there as i couldn't find an online application. Then they will message you when your appointment is due. It should take an average wait time of 1-2 hours for them to call you.