How can I install Facebook on my iPhone 6

You can’t. Meta has decided that people with a phone as old as an iPhone 6 can’t install the current version Facebook app. You can still access it using Safari. You can go to the App Store, find Facebook, and see if they will allow an older version to be installed.

Open the App Store on your phone. Search for Facebook then tap Get. You will be prompted to log in or create a new account.

Edit - I stand corrected per Lawrence Finch’s response.