How much does it cost for a replacement battery for an iPhone 6s and can just anyone complete that task?
iPhone 6s Plus, iOS 15
Make sure that you have the battery replaced at an Apple Store or authorized Apple Service shop. They are the only locations that will use a genuine Apple battery for the iPhone.
Other "knock off" batteries will not display some or all of the important battery settings like Battery Health.
According to Apple's estimate, it will cost $49 to replace the battery on your iPhone 6S.
Battery replacement is very common and I would think most Smart Phone Repair shops can do the job. However, it is important that you insert an original battery, this will ensure that all functions work as they should + the safety perspective.
You can read more and see prices for iPhone battery replacement at Apple here:
iPhone Battery Repair & Replacement - Apple Support
Battery Replacement
