Mine is iPhone 6s ,but unable to download the latest update

Use internet speed over 15 Mbps (both download and upload) speed for error free update. Avoid latching on to Hotspot or WiFi Extenders

VPN or proxy connections might prevent your device from contacting the update servers.

• Get help with over-the-air iOS updates - Apple Support

• Remove and download the iOS update again

If you still can't install the latest version of iOS, try downloading the update again:

1. Go to Settings > General > [Device name] Storage.

1. Find the iOS update in the list of apps.

1. Tap the iOS update, then tap Delete Update.

1. Go to Settings > General > Software Update and download the latest iOS update.

If you don’t see the update in the list of apps or if the issue happens again, you need to update your device with iTunes.

Try updating iOS after each step.

• Try Forget the WiFi network and add again

• Try another WiFi network 

• Toggle sets automatically off and then on

• Reset your network settings

• Force restart your device

• Remove any beta profiles, if applicable

• If everything fails: - Use iTunes or Finder (for MacOS Catalina+) to update your device instead of over-the-air

• Get help with iOS update and restore errors - Apple Support

• If you see an error when you update or restore your iPhone, iPad

These are the iOS options based on iPhone Model --> Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

You need to Identify your iPhone model - Apple Support

• iOS 14.3 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max
• iOS 12.5 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)
• iOS 10.3.3 —> Final release supported on iPhone 5C and iPad (4th generation)
• iOS 10.3.4 —> Final release supported on iPhone 5 and iPad (4th generation) Wi-Fi + Cellular
• iOS 9.3.6 —> Final release supported on iPhone 4S, iPad 2 Wi-Fi + Cellular, iPad (3rd generation) Wi-Fi + Cellular, and iPad Mini (1st generation) Wi-Fi + Cellular
• iOS 7.1.2  —> Final release supported on iPhone 4.
• iOS 6.1.6 —> iPhone 3GS and iPod Touch (4th generation) only
• iOS 4.2.1 —> Final release supported on iPhone 3G and iPod Touch (2nd generation)
• iOS 6.1.5 —> Final release supported on iPhone 3GS, and iPod Touch (4th generation)
• iOS 3.1.3 —> Final release supported on iPhone (1st generation) and iPod Touch (1st generation)