How to update 15.6

Go to Settings/General/Software Update and follow the instructions. If it doesn’t offer to update to 15.6 report back what it says, or post a screenshot.

You can't update iOS 15.6.

You can update to iOS 15.6 if your iPhone is qualified

Settings > General > Software Update on your iPhone. More info here:

Update your iPhone, iPad, or iPod touch - Apple SupportOfficial Apple Support › en-us