Can I update my iPhone 6s to iOS 15.8?

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 -->

• iOS 17.1.1 —>
  • A16 Bionic (4 nm) - 15 (6GB RAM) / 15 Plus (6GB RAM)
  • A17 Pro (3 nm) - 15 Pro (8GB RAM) / 15 Pro Max (8GB RAM)
  • Chipset A12 - XR (3GB RAM) / XS (4GB RAM)/ XS Max (4GB RAM)
  • Chipset A13 - SE (2020) (3GB RAM) / 11 (4GB RAM)/ 11 Pro (4GB RAM)/11 Pro Max (4GB RAM)
  • Chipset A14 - 12 Mini (4GB RAM) / 12 (4GB RAM) / 12 Pro (6GB RAM) / 12 Pro Max (6GB RAM)
  • Chipset A15 - SE (2022) (4GB RAM) / 13 Mini (4GB RAM) / 13 (4GB RAM) / 13 Pro (6GB RAM) / 13 Pro Max (6GB RAM)
  • Chipset A16 - 14 (6GB RAM)/ 14 Plus (6GB RAM) / 14 Pro (6GB RAM) / 14 Pro Max (6GB RAM)
• iOS 16.7.2 —>
  • Chipset A11 - 8 (2GB RAM) / 8 Plus (3GB RAM) / X (3GB RAM)
• iOS 15.8 —>
  • 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)

iOS 15.8 is as far as the iPhone 6s can go. No more updates for the iPhone 6s.

If you want to run iOS 16, you need an iPhone 8 or higher model.

If you want to run iOS 17, you need an iPhone 11 or higher model.

An iPhone 6s, iPhone 7, or iPhone SE 1 isn't capable of 16 or above; they require an A11 or better CPU.

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