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. 

If your iPhone is on iOS 12, it's highly likely you don't have a 6s Plus, and instead own an iPhone 6.

To be sure, please go to Settings > General > About > Model. Tap it to reveal a number like A2236 and report back.

If you actually own an iPhone 6 Plus, the maximum iOS version it can run is iOS 12.5.7.