How can I update my iPhone 6s Plus

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. 

Check all of these carefully:-

1. you are on a good Wi-Fi, (from 15.4 iPhone 7 up, you may be offered cellular update)  and  that the router is connected to the internet.
2. Plenty of spare storage – 6GB at least.
3. on charge with more than 20%, OR 
4. off charge with more than 50%
5. low power mode turned off. 
6. Check you have no Beta profile installed. (Check Settings, General, VPN & Device Management)
7. no VPN installed (Check Settings, General, VPN & Device Management)

Resources:-

Update iphone

Update your iPhone or iPad - Apple Support

Learn how to update your iPhone via recent Macs here:-->

Update your iPhone, iPad or iPod touch with the Finder – Apple Support (UK)

or older Macs and PC here:-->

Update your iPhone, iPad or iPod touch with iTunes – Apple Support (UK)

KB Article - Iphone will not update

If your iPhone or iPad won't update - Apple Support

The iPhone 6S+, released by Apple over seven years ago, cannot be updated beyond iOS 15.7.x.