So, you have a plugin requiring that you make a change to one of your WordPress theme templates, or you just want move something around in your theme or alter your CSS. You’ve arrived at Appearance -> Editor as instructed, but there’s no “Update File” button.
This is unfortunately another one of those common annoyances related to your hosting provider’s server configuration that can take you off guard and lead to plenty of confusion. Fortunately, it’s easy to fix (or work around).
Of course, if neither of those methods are working out for you, please stop by the WordPress Support Forums for further assistance.