How to update my iPhone from iOS 12 to iOS 16?

You don't write which iPhone you have, but below you can see your options.

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

If you want to update from iOS 12 to iOS 16, you must update from a computer.

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

iOS 16 and iPadOS 16 are compatible with these devices - Apple Support

If you device its compatible, follow these steps:

Update your iPhone or iPad - Apple Support