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Apple released a statement detailing the following:

• Apple is reducing the price of an out-of-warranty iPhone battery replacement by $50 — from $79 to $29 — for anyone with an iPhone 6 or later whose battery needs to be replaced, available worldwide through December 2018. Details will be provided soon on apple.com.
• Early in 2018, we will issue an iOS software update with new features that give users more visibility into the health of their iPhone’s battery, so they can see for themselves if its condition is affecting performance.

Apple Will Replace the Battery in Your iPhone 6 or Later Even if It Passes a Genius Bar Diagnostic T...

If your needing your battery replaced here are a few suggestions to begin the process:

• Make an appointment via the Apple Store app: Apple Store on the App Store
• Backup your device via iTunes/iCloud