How to update my iPhone 6plus iOS 14

If your phone is not a 6s Plus or higher, then you aren't going to be able to run it. If you just have the iPhone 6 Plus, then it's not going to be able to run it. You can check iOS 14 - Apple to get a list of the devices that are compatible, but anything 6s or higher can run it. If your device can run it, you go to Settings > General > Software Update and let it check for updates. If you get "No Updates Available" then your device is running the highest version that it is capable of running and will not update any further.

If you read the reply above yours....You'll see the answer to your question. If you are running a 6 Plus and not a 6s Plus, you are not going to be able to update to iOS 14.

I want to update my iPhone 6plus from 12 to 14