How to I get ios 12 for my iPhone 6s

Use a computer or the Software Update function in Settings to update to 15.5. It's not possible to update to an intermediate iOS version.

(218746)

Check iPhone type & iOS version -> 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 your iPhone is eligible for an update, you can check by reading this Apple article:

iPhone models compatible with iOS 15

iPhone models compatible with iOS 15.5 – Apple Support (UK)

Then 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

You need about 7 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.