How do I update iOS for my iPhone 6?

Unless you have not updated your iPhone 6 in a very long time.......it is already running iOS 12.5.7.

That was the last....and final.....update for the iPhone 6. No more updates will be available for the iPhone 6.

The iPhone 6 cannot run iOS 13, 14, 15, 16, or 17, so you are going to need a new phone if you want to run any current iOS version.

If your “6” is running iOS 12 (specifically 12.5.7) …

… it’s already updated as far as it is capable.

If you require a more modern and capable iOS, you’ll also need a newer and more capable iPhone.