I want to update to iOS 15

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 had the beta version of iOS 14 on your phone you must remove the beta profile before it will update to iOS 15.