Editing wordpress theme without edit the origin theme file. There are many way to castumize wordpress theme. One of them is using child themes ( http://codex.wordpress.org/Child_Themes ). As wordpress said, child themes are the recommended way of making modifications to a theme if you want your modifications are still there. To start editing please follow Codex wordpress on http://codex.wordpress.org/Child_Themes , or follow this steps :
*this example using fastfood theme as original theme.

• Create directory in themes directory, for example fastfood-child
• Create style.css and import the fastfood theme style, look at the code below

/*
Theme Name:     fastfood Child
Theme URI:      http: //mendoan.us/
Description:    Child theme for the fastfood theme 
Author:         securesh
Author URI:     http: //kencur.net/about/
Template:       fastfood
Version:        0.1.0
*/

@import url("../fastfood/style.css");

/*
body {
	background: #C0C0C0;
}
*/

• To support RTL languages, add rtl.css file to your child theme

/*
Theme Name:     fastfood Child
Template:       fastfood
*/

@import url("../fastfood/rtl.css");
[/sourcecode]

For example if you want add a meta tag from search engine verification and add the favicon or icon for your web : create functions.php in child theme directory, and paste this code

<?php
function favicon_link() {
?>
<link rel="shortcut icon" type="image/x-icon" href="your image" />
<?php
}
add_action('wp_head', 'favicon_link');
function seoverify() {
?>
<your meta code>
<meta name="msvalidate.01" content="your validation" />
<?php
}
add_action('wp_head', 'seoverify');
?>

To use the child theme just activate your child theme from wp-admin.
You are don :)