I want update my iOS to 15.7.9

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.

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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Halp