When we will update ios 12 in iphone 6 and how

Animi & Measure app is not supported in iPhone 6.

The Measure app uses augmented reality (AR) technology to turn your iOS device into a tape measure. You can quickly gauge the size of objects, automatically detect the dimensions of rectangular objects, and save a photo of the measurement for later.

First, make sure that you're using iOS 12 on iPhone SE, iPhone 6s or later, iPad (5th Generation), or iPad Pro. Also make sure that you're in a place with good lighting.

Sorry, I forgot this line on my previous reply:

Update your iPhone, iPad, or iPod touch - Apple Support

Giulio

Today (but we don't know at what time, that can be different from country to country).

Regards

Giulio