If you want to flash a new firmware on your Galaxy Note 2 or even root it, you will have to install custom clock work mod recovery (CWM) in it. Keep in mind that after install custom clockwork mod, you are voiding the warranty. So in future, if you wish to send it for warranty, you must flash galaxy note 2 stock firmware. Anyway, here are the instructions for downloading and installing CWM.
-Download CWM recovery file from here and reboot device to download mode
-Open odin and select downloaded file for PDA tab
-Connect device to PC, and wait till device successfully added to com port