How do I update my iPhone from iOS 12.5.7 to iOS 15.8?

An iPhone 5S, 6, or 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.

Your iCloud account has nothing to do with this.

• 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 15.8.4

• If you have an iPhone 8 or X, you can update to 16.7.11

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.4.1

You already asked this question within the other similar topic. Asking again won't change the answer.

Nope.