How can I update

latavia291 wrote:

How can I update a iPhone 7 to 15.0

You can only update to the latest version, 15.7.3. You can update your iPhone by going to Settings -> General -> Software Update and installing the particular 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.