I want to update my IPhone to iOS 16

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 7, you can update to 15.7

• If you have an iPhone 8 or higher, you can update to 16.0.2

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support

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

• iOS 15.7 —> 6S / 6S Plus, SE (1st Gen), 7 / 7 Plus,
• iOS 15.7 or iOS 16.0.2 --> 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

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.