How can I replace Apple Emoji font on El Capitan from Sierra? I've got my hands on Apple Emoji TTC from Sierra and I'd like to install it on El Capitan, but since Apple Emoji TTF on el Capitan is a system file, it cannot be moved into trash, disabled or updated. Is it a possible operation at all?
I cannot upgrade to Sierra, but I'm also fed up with seeing symbols instead of emojis because my OS does not recognise them.
Even if you replace the system emoji font, this won't help with all emojis. This is because some emojis are combinations of characters and this may not be interpreted correctly if the codepoints are new.

If you still want to replace the emoji font, you need to disable System Integrity Protection to replace system files.


  To disable System Integrity Protection, you can do the following:
  
  
  Boot into the Recovery HD by restarting whilst holding ⌘R.
  Open Terminal (from the Utilities menu).
  Run the following command in Terminal:

csrutil disable

  Restart.
  


Source: Disable OS X El Capitan Rootless and permit write actions to System Integrity Protection locations