To have the battery of an iPhone 4S replaced, go to the iPhone Support section of Apple.com, select "Repair & Service," then "Battery & power" and request battery service by clicking "Start a service request." On the next page, choose "Battery, Power & Charging," and then select the appropriate subcategory.
You may opt to send your iPhone 4S to Apple for repair or replacement or visit an Apple Retail Store or an Apple Authorized Service Provider. Battery service is free of charge if your Apple One Year Limited Warranty is still valid. Enter your iPhone serial number to find out if you still have replacement coverage for a faulty battery.
If your warranty has expired, the cost of battery service for all iPhone models is $79 and an extra $6.95 for shipping, as of 2015. Back up all data on your phone before sending it to Apple for repair to avoid losing important data.
After sending the phone to Apple or bringing it to an Apple Retail Store, you may get a replacement or a fixed iPhone within three to five business days. Apple technicians perform diagnostic tests to check for a defective battery. If you receive a replacement phone, the device is likely new or comparable to a new iPhone in terms of dependability and performance.