How to I get the iOS 16 update

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. 

Settings > General > Software Update > Upgrade to iOS 16

Update your iPhone or iPad - Apple Support

iOS 16 is available on iPhone 8, 8 Plus and newer.