how to update?

You do not write which iPhone you have, so it is not certain that you can update. You can see your options below:

• iPhone 5S/6 can be updated to: iOS 12.5.7
• iPhone 6S/7/SE 1.Gen can be updated to iOS 15.7.3
• iPhone 8 or newer can be updated to: iOS 16.3.1

You can check your current version here:

Settings → General → About

You can download and install software updates here:

Settings → General → Software Update

Remember to connect to Wi-Fi and a charger. Your iPhone must have more than 50% power to update without a charger connected.

You can find more useful information here:

Update your iPhone or iPad - Apple Support

If your iPhone or iPad won't update – Apple Support (UK)

If you want to update your iPhone via a PC or Mac, see here:

• Mac: Update your iPhone, iPad, or iPod touch with the Finder - Apple Support
• PC (iTunes): Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

These are the instructions:--> Update your iPhone, iPad, or iPod touch but before proceeding, check if your unidentfied iPhone model supports more recent iOS versions, as per this compatibility list: