I have a Log out link in a menu which is pointed to user/logout. I'm trying to edit the name "Log out" to something else like "Sign out". How can I do that?
You could use the String Overrides module to globally change "Log out" to "Sign out".
If you are going the route of changing "log out" to "sign out" globally, you can achieve that without adding a module. Look near the bottom of settings.php for the string overrides section. There are instructions there and examples.