how to update an iphone?

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Based on the tags on your post, you have an iPhone 7. This device is not capable of running iOS 16 or higher. Your device can currently run up to iOS 15.7.3. If you need iOS 16 or higher, you will need a new device.

Your iPhone is too old and would not be compatible with the newer software updates.

Click ▶︎ iPhone compatibility with iOS 16 for the complete list.

The time has come:

Shop one-on-one with a Specialist at an Apple Store.

Click ▶︎ Reserve a Shopping Session. 🛒