How can i update my iOS 12.4
How can i update my iOS 12.4 TO 15
iPhone 6s, iOS 12
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You might encounter many problems to update that device wirelessly, since the iOS version running on it is very old.
I suggest that you try to update it via computer (if you don't have a computer borrow one from your friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider), then learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.
If you still get no help, contact Apple Support directly, using this link:--> Get Support
Regards
Giulio