You need to go to a franchise store designated by Apple to change the battery. Before replacing it, it is best to check the mobile phone information on Apple's website to see if it meets the conditions for free replacement.
Free battery replacement service:
Both Apple Store and Apple officials authorize free battery replacement service after sale, provided that there is something wrong with the mobile phone after testing.
It should be noted that the free battery replacement service of Apple Store is looser than that of authorized stores. In the Apple Store, if there is a problem with the battery of the mobile phone, the official can provide the service of replacing the battery for free, while the official authorized store strictly depends on the time, requiring users to provide the purchase certificate and warranty card, both of which are indispensable.