Requirements for iPhone 6 battery replacement
From the Apple site
https://support.apple.com/en-us/HT201557
Before you bring in your device
Follow these steps before you take your device to an Apple Store, Apple Authorized Service Provider, or carrier. If your device won't turn on or respond, finish as many steps as possible:

Back up your iOS device.

Have your Apple ID password ready. For some
repairs, you'll need your Apple ID password to erase your device and
turn off Find My iPhone.*

Bring your sales receipt (if possible), in
case your service requires proof of purchase.

Bring your device and any accessories that you need help with.

Bring a form of personal identification, like a driver’s license, passport, or other government-issued ID.

If you haven't already, call to check whether you need an appointment before you go to a service location. For example, schedule a Genius Bar reservation before you go to an Apple Store.


Before you send your device
Follow these steps before you send your device in for repair or replacement. If it won't turn on or respond, finish as many steps as possible:

If you need to send in an iPhone that's paired with an Apple Watch, unpair your devices. When you unpair, your iPhone will create a new backup of your Apple Watch. You can use the backup to restore your Apple Watch when your iPhone returns from service.
Back up your iOS device.
Go to Settings &gt; Messages and turn off iMessage.
Erase your device. Go to Settings &gt; General &gt; Reset &gt; Erase All Content and Settings. If asked, enter your passcode and Apple ID password. If you can't erase your device because it won't power on or isn't responding, you can erase it from iCloud.com. Learn how.
Make sure that Activation Lock is disabled.
Remove any SIM card, case, or screen protector from your device and keep your cable and charger.
Now your device is ready to send. If you arranged a repair request, you should have received instructions for how to mail in your device. If you didn't get instructions or you haven't set up a repair request, contact Apple Support.

You can find additional answers here: https://support.apple.com/iphone/repair/battery-power