How to update my iPhone 6s?

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.x. If you require a higher level of iOS to purchase and run a particular app, you will need to purchase a new, or newer phone.