Hi Shivank,
What kinds of changes do you need doing to your footer?
If it's just CSS changes, e.g., background color, text color, text size, etc., then the easiest step to take is to download a plugin like http://wordpress.org/plugins/simple-custom-css/ where you'll put your style changes.
If you need more structural changes, then you'll have to create a child theme (http://codex.wordpress.org/Child_Themes) and duplicate footer.php from the parent theme into your child theme. There you can make your changes.
It's important that you don't apply modifications directly to your theme, otherwise, the next time your theme gets updated, you'll lose all your changes. And it's recommended that you always update your theme (and plugins, and WordPress itself).
If you let me know exactly what you'd like done to the footer I could be of much more help to you.
All the best,
Cheers!
- Maria Antonietta