How to update IOS 12.5.6

What happens when you go to Settings -> General -> Software Update -> Download and Install?

Do you get any error message?

If it is an iPhone 6 you can update to iOS 12.5.6.

If it is an iPhone 6S you can update to iOS 15.7.2.

You need about 6 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

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

If it is running iOS 12.5.6, it can not be an iPhone 6S. Instead, it has to be an iPhone 5S, 6 or 6 Plus. Neither can be updated further.