How to update my iPhone 6 Plus to iOS 15?

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 15.8.2

• If you have an iPhone 8 or X, you can update to 16.7.7

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.5.1