I have iPhone 6 Plus. Is it possible to install iOS 14 on it?

If your phone is at iOS 12, it is up to date for that phone and it cannot support the newer iOS versions. If you need the newer iOS versions, then you'll need to purchase a newer, more modern iPhone.

NO......and just how do you think you can open an iPhone and install parts NEVER designed for the phone? Not to mention the phone lacks the most important part, which is RAM and there is NO WAY to increase the Ram on an iPhone 6. You'll likely destroy it and Apple won't help you at all.

Face it. You have an out of date, old device, which needs to be replaced with a newer, more modern iPhone.

No, its not. iPhone 6 lacks the hardware required for the newer iOS versions.

Sorry - no

How do I update my iOS above 12? It tells me my iOS IS up to date and won't show higher versions.

You don't

An iPhone 6 can't be updated beyond 12.5.4, it doesn't have the hardware needed to run a higher iOS.

if I install a faster chip like the A9, will Apple upgrade my operating system?

Good luck with installing that CPU or even finding one, but you will also have to double the RAM that is on your logic board.