Why not update ios 16.1 in my mobile

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

• iOS 15.7.1 —> 6S / 6S Plus, SE (1st Gen), 7 / 7 Plus,
• iOS iOS 16.1 --> 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

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.
2. Find the iOS update in the list of apps.
3. Tap the iOS update, then tap Delete Update.
4. 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 to Forget the WiFi network and add it 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

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.