Download iOS 16

The best that the iPhone 6 can do is iOS 12.5.7. Nothing higher than that.

If you need to run higher iOS versions, you need a new phone.

godfred229 wrote:

How to download iOS 16 for iPhone 6

You do not. The iPhone 6 is not compatible with iOS 13 or higher.

Sorry, you can't. The 6 is not capable of handling iOS 16.