How to update iOS 12.4 on 6s

Check iPhone type -> Go to: Settings -> General -> About.

If it is an iPhone 6 -> iOS 12.5.5 is the last security updates for iPhone 6, -> released 23 september 2021.

If it is an iPhone 6S you can update the iPhone to iOS 15, see this Apple article:

iPhone models compatible with iOS 15

iPhone models compatible with iOS 15.4 – Apple Support (UK)

Go to: Settings -> General -> Software Update.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

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

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support