Software update issue

Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone. 

• Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 6GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.

One thing that you should know and confirm and that is whether you have an iPhone 6s or do you have an iPhone 6 (Without the S) This is important because an iPhone 6 can only be updated up to iOS 12.5.6 and NOTHING HIGHER. You need an iPhone 6s to update higher than that to iOS 15. Double check which iPhone you have.

Open Settings ➔ General ➔ About ➔ Model Name: What does it state there?

Axel F.

iPhone 6s is not qualified to receive the iOS 16 update, if that might what you are asking about.