How do I update iOS 15 in my iPhone 6

An iPhone 6/6 plus can't be updated beyond 12.5.7, it doesn't have the hardware required to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.3

Your iPhone 6 no longer receives the latest iOS software updates and cannot be updated past iOS 12.5.7. You will need a purchase a newer iPhone to run iOS 15, or ideally the latest iOS 18.