How can I update my iPhone 6s version 13.3 to 15.7

You're probably behind to much for the on-device update to work. You can try going to Settings->General->Software Update and see if it will let you update.

If it issues an error, you will need to update on a computer, Mac or PC, with iTunes or Finder. Follow the steps in the following links:

click here-> if you're using a Mac with macOS Catalina or later

click here-> if you're using a Mac with macOS Mojave or earlier or a Windows PC

See instructions in: Update your iPhone or iPad - Apple Support

Use a computer or the Software Update function in Settings. If you encounter a specific issue, describe it in this thread.

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