FTP SFTP, SSH, or code editor application — like Coda or Notepad++, is always a better idea than using the built-in editor inside WordPress.
The reason being is that WordPress doesn’t check for fatal errors in your code before saving it. It becomes very easy to lock yourself out of WordPress if you create an error in your code and suddenly your Dashboard goes blank.
STOP EDITING YOUR FUNCTIONS.PHP FILE IN YOUR WP DASHBOARD -> APPEARANCE -> EDITOR PEOPLE!
— Scott Basgaard (@scottbasgaard) July 4, 2012
It happens all too often and it causes people a lot of issues.
So before you start editing the code, make sure you’re doing the editing in an application that won’t lock you out.