How much does battery cost for iPhone 6
How much is a new battery for iPhone 6s
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You can make an appointment at the Apple Retail Store - Genius Bar and take it in for service. You can also look for an Apple Store - Find a Store - Apple. You will find respective repair costs from the appropriate link: iPhone Service Support and Costs.
Battery replacement should be around $79. However, you should see iPhone 6s and iPhone 6s Plus Service Program for No Power Issues and iPhone 6s Program for Unexpected Shutdown Issues.