Can I update iOS 6 to iOS 12?

Yes, an iPhone 6 can run 12.5.6.

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

It would help if you can provide us with the exact model of your iPhone? For example, if it is an iPhone 3GS, then it can only be upgraded to iOS 6.1.6. To upgrade to iOS 12.5.6, you would need an iPhone 6 or 6 Plus.

No. A model with 6 installed will be an iPhone 3GS through 5, and none of those are capable of 11 or above.

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