Software update

Are you 100% certain that you have an iPhone 6s Plus and not an iPhone 6 Plus? An iPhone 6 Plus cannot be updated past iOS version 12.5.1. You already have that version and are unable to update beyond it and therefore I am asking.

Open Settings ➔ General ➔ About ➔ Model Name : Here it should tell you which iPhone model you have.

Axel F.

You have a 6 and not a 6s.

No updates for your device.

because your iPhone hardware is not compatible with anything newer than iOS 12

Update your iPhone, iPad, or iPod touch - Apple Support

If you're having issues, try this: About iOS 14 Updates - Apple Support

You are welcome.

Axel F.