Update iOS 12.0
How to update 12.0 iOS when I install it later is answer
iPhone 6s
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Update your iPhone, iPad, or iPod touch
• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5
• If you have an iPhone 6S through 13, you can update to 15.4.1
Go to Settings > General > Software Update.
Update your iPhone, iPad, or iPod touch using WiFi -
Update your iPhone, iPad, or iPod touch - Apple Support
Update using your computer -
Update iOS on iPhone - Apple Support
If your iPhone, iPad or iPod touch won’t update
If your iPhone, iPad or iPod touch won't update – Apple Support (UK)