How can I update iOS 16.6 on iPhone 11?

You can only install the current version of ios for your device.

The current version for the iPhone 11 is 18.3.1

The implication of your question is that you are running something older than ios 16. If that’s the case, an over the air update (settings>General>Software Update) will probably fail.

You need to plug the phone into your computer and update that way instead to avoid problems.

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

You can't install iOS 16.6 on an iPhone 11. iOS 16 (any version of iOS 16) hasn't been signed in a couple years. You CAN, however, update your iPhone to the most current version, which is iOS 18.3.1.

If you haven't updated your phone in a few years, you'll need to use your computer to update your iPhone.

See the following two links depending on whether you have Windows or Mac:

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support