I want to update my phone

An iPhone 6s cannot be updated beyond iOS 15.7.2. If yours is not already updated to that, see: Update your iPhone or iPad - Apple Support

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.