How do I update iPhone 7 to latest iOS version?

That will not be possible. Your iPhone 7 no longer receives the latest iOS software updates and will not update past iOS 15.8.3. If you want to update to the latest iOS version, 18, you will need to purchase a newer iPhone that actually supports it.

Apple does not usually say the specific reason "why".

Two reasons that come to mind why the iPhone 7 would be incompatible with iOS 18 are:

• Insufficient RAM. The iPhone 7 has only 2 GB of RAM. All iPhones that can run iOS 18 have at least 3 GB of RAM, and it looks like those are stragglers which barely made the cut. Most iPhones compatible with iOS 18 have 4, 6, or 8 GB of RAM – that is say, two, three, or even four times as much as your iPhone 7.
• No Neural Engine. All iPhones that can run iOS 16, 17, or 18 have Neural Engines, to help speed up machine learning type computations. Your iPhone 7 has an A10 Fusion processor chip that is old enough to lack one.

jigs274 wrote:

iPhone update karna h mera phone pay kuch bhi nhi chal raha

Automated translation (from Hindi): "I need to update my iphone nothing is working on my phone"

You cannot update an iPhone 7 to iOS 16 or higher.

If "phone pay" is meant to be a reference to making or receiving UIPI payments, the people who make the rules for the UPI system have apparently decided that you need to be running at least iOS 17 to make or receive them on an iPhone. All UPI app vendors must comply. Even those like PhonePe that have other, non-UPI functions that might still work on lower versions of iOS are not allowed to process UPI payments on anything less than iOS 17.

In effect, the people who make the rules for the UPI payment system have abandoned support for your iPhone 7. If you want to keep making or receiving UPI payments on a phone, you will need to buy a new phone.

Athonypompilaino wrote:

Why can I update it

Due to hardware limitations.

iPhone update karna h mera phone pay kuch bhi nhi chal raha

This may help you --> Apps that require higher iOS versions than your device can be updated to... - Apple Community