Unable to update my iPhone 6s software

Why not, what happens when you try? Below is a list of over the air update requirements for your iPhone written by Community member Lawrence Finch. Keep in mind you can update your iPhone 6s to iOS 15.8.3 and no further.

• You need at least 10 GB of free storage for iOS 16 and later or 6 GB for iOS 15 or lower; more is better
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone that supports cellular 5G (12, 13, 14, 15 or SE 3rd edition) and has a 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode and must have a cellular data connection
• You cannot still have a beta of a previous version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.

You cannot pick and choose an iOS version. What you get is Settings > General > Software update > [All options are available here]

You need to correctly Identify your iPhone model - Apple Support

These are the iOS options based on an iPhone Model

(Updated Jan 4, 2025 12:05 PM)

• iOS 18.3.1 —> iPhone models compatible with iOS 18
• iOS 16.7.10 —> iOS 16 and iPadOS 16 are compatible with these devices
  • Chipset A11 - 8 (2GB RAM) / 8 Plus (3GB RAM) / X (3GB RAM)
• iOS 15.8.3 —> iOS 15 is compatible with these devices.
  • Chipset A9 - 6S (2GB RAM) / 6S Plus (2GB RAM) / SE (1st Gen) (2GB RAM)
  • Chipset A10 - 7 (2GB RAM) / 7 Plus (3GB RAM)
• iOS 12.5.7 —>
  • Chipset A7 - iPhone 5S (1GB RAM)
  • Chipset A8 - iPhone 6/6 Plus (1GB RAM)
• iOS 10.3.4 —>
  • Chipset A6 - iPhone 5 (1GB RAM)
• iOS 10.3.3 —>
  • Chipset A6 - iPhone 5C (1GB RAM)
• iOS 9.3.6 —> iPhone 4S (512MB RAM) - Chipset A5
• iOS 7.1.2 —> iPhone 4 (512MB RAM)- Chipset A4

Know how to...

• Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
• Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

You can update an iPhone 6s to the latest release of iOS 15 – currently iOS 15.8.3.

About iOS 15 Updates - Apple Support

Updating iOS: common issues and solutions - Apple Community

You cannot update an iPhone 6s to iOS 16 or later. For the iPhone 6s, iOS 15 is the end of the line.

Tips on what to do:

Check Compatibility-> ensure iPhone6s can update to iOS 15.8.3

-> free up some of that space & make sure you have enough storage space

-> go to Settings-> General -> [device name] Storage and remove any unnecessary apps or content

• Ensure stable Wi-fi internet connection
• Charge your iPhone -> make sure phone is connected to a power source -> update requires sufficient battery
• Force restart: if the update is stuck, try restarting your iPhone. For iphone 6s-> hold both "home" button and "Power" button until the Apple logo appears
• Update using a computer-> use if over-the-air update fails
• Recovery Mode: if the update remains stuck, you can use "recovery mode" to reinstall iOS without erasing data.
• Connect your iPhone to a computer, open "finder" (on macOS) or iTunes (on Windows), and enter Recovery Mode
• DFU Restore-> As a last resort, you can perform a device firmware update (DFU) restore-> this will completely reinstall iOS but will erase all date

+ If none of these steps work, you may want to contact Apple Support or visit an Apple store for further assistance -> apple.com or simplymac.com

Probably you are seeking to update becuase --> New Apps that require higher iOS versions than your device can be updated to... - Apple Community