Apple noted that recent iOS updates caused a slowdown in older iPhone performance, but stated this was required to prevent random phone shutdowns caused by an older battery.

Apple responded to the public feedback on the battery controversy by releasing this statement.  Apple also lowered the price to replace an iPhone 6 and higher battery from $79 to $29 worldwide through December 2018.  Apple iPhone technical support can be requested at getsupport.apple.com or by calling (800) MY–IPHONE (800–694–7466).  AppleCare plan status and eligibility may be checked by entering the device serial number at checkcoverage.apple.com.

Apple online support for all of its products may be found at support.apple.com/sitemap.

Third-party or DIY iPhone repairs will void your warranty if not authorized by Apple.  However, website iFixit has no affiliation with Apple but maintains iPhone repair instructions and sells DIY battery replacement kits for various iPhone models.