How to update software if iPhone 6s Plus from 12.5to upper software

While updating iOS is not mandatory, but updating your iOS software provides essential security patches, bug fixes, and enhancements that fortify your device's defenses against potential vulnerabilities. By staying up to date, you ensure that your device benefits from the latest security measures implemented by Apple, reducing the risk of unauthorized access, data breaches, and other malicious activities. Regularly updating your iOS not only enhances the overall security of your device but also helps maintain the privacy and integrity of your personal information. Therefore, while it may not be obligatory, updating iOS is strongly recommended to ensure optimal protection for your device and sensitive data.

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 iPhone Model -->

• iOS 16.5.1 —>
  • Chipset A11 - 8 (2GB RAM) / 8 Plus (3GB RAM) / X (3GB 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 15.7.7 —>
  • 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)