How to download iOS 15.7.3 on iPhone?

Lawrencebrima352 wrote:

I want to download iOS 15.7.3

An iPhone 6 is supported through iOS 12, and no higher. There are no hardware upgrades. Your options for newer iOS are a newer (used, refurb) or new iPhone. iPhone 8 and newer are required for the current iOS 16 version.

If you have an iPhone 6 as indicated, you can not.

That model can be updated to iOS 12.5.7, no further.