I want to update my phone

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. 

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