Hi cem1,
If you like to customize the code then you need to create a child theme before making any changes in the theme. So that while updating to our new version you will be safe from data loss while updating to new version. If you make a direct change on the code then each and every time you make changes in the code, the same thing you have to do again.
The file is inside theme folder( interface)-> inc -> widgets-> interface_widgets.php, You need to go through this class on line no 200
class interface_service_widget extends WP_Widget {
Thank you!