Software update for iPhone 6s

A 6S can only be updated to iOS 15.7

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 7, you can update to 15.7

• If you have an iPhone 8 or higher, you can update to 16

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support