functions.php is generally in the root of your theme, not the plugin e.g.
wp-content/themes/[theme name]/functions.php
If you are going to start editing your theme’s files, I suggest you use a child them – which is super simple to create, see this: http://codex.wordpress.org/Child_Themes. You can then create a new functions.php in the child theme folder and edit it. This will ensure any changes are not overwritten when you update your theme.
Note: a child theme functions.php is IN ADDITION to the parent theme functions.php, so no need to copy the contents of the parent functions.php – just create a new one.