I want to upgrade my iOS to 15.7

These are the iOS options based on iPhone Model --> You need to Identify your iPhone model - Apple Support

• iOS 15.7 —> 6S / 6S Plus, SE (1st Gen), 7 / 7 Plus,
• iOS 16.0.3 --> SE (2nd and 3rd Gen), 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max, iPhone 13, 13 Mini, 13 Pro, 13 Pro Max, 14, 14 Plus, 14 Pro and 14 Pro Max
• iOS 12.5.6 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 

Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

If you have an iPhone 6 then you can update to iOS 12.5.6

If you have an iPhone 6s then you can update to iOS 15.7

If you have an iPhone 7 then you can update to iOS 15.7

If you have an iPhone 8 or a newer model then you can update to iOS 16.0.3

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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
• Ensure that you do not have a beta profile installed from a previous version
• 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.