How to update my iPhone to the latest iOS?

If your iPhone is too old, it would not be compatible with the newer software updates.

➢ iPhone compatibility with iOS 16.

Follow these steps, testing after each step:

• Software Update.
• Update iOS on iPhone.
• If your iPhone won't update.

Need more help?

• Call Apple Support at 1-800-APLCARE. 📞
• International Support. 🌐
• Contact Apple Support. 📝
• Live chat using Messages. 💬

Click ▶︎ iPhone User Guide. 📖

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.