Why not working iPhone 6s plus ios 14
Not available ios 14 in iphone 6s plus
Update your iPhone, iPad, or iPod touch
• 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
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