How to request ios 14 update for iphone 6s plus?

You can't. If you want to upgrade, you can only use the current version which is 15.6.1.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S through 13, you can update to 15.6.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

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

Update using your computer -

Update iOS on iPhone - Apple Support

you can't, when ios versions stop being signed there is no way to install them any longer

this is the only version signed for your device iOS 15.6.1 (19G82) for iPhone 6s

you can try contacting apple Support ->  Get Support (apple.com)

and or give them feedback -> Product Feedback - Apple

That’s not an option. The only thing you can update to is the current version for that model. 15.6.1.