iPhone 6 plus not showing ios 13 and above for upgrade

I have an iPhone 6 plus device. This is currently on ios 12 which is not the latest ios so I wanted to update it to ios 14 but the phone says its already up to date with ios updates!

As per https://support.apple.com/en-us/HT211348 my iPhone 6 plus should be compatible with ios 14

yet it does not let me update it. thinks ios 12 is latest and my phone is already up to date! not sure why? and how can I update ios given this issue?