How can I use the iOS 15 version?

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. 

If you have an iPhone 6 - Then you can only install iOS version 12.5.6 and nothing higher.

If you have an iPhone 6s or an iPhone 7 - Then you can only install iOS version 15.7 and nothing higher.

If you have an iPhone 8 or a newer model iPhone - Then you can install iOS version 15/16.

Which iPhone do you have? Which iOS version is currently installed on it? What happens when you attempt to update your iPhone?

Axel F.