How to locate the index.php file??

I am trying to add a static welcome message to my blog home page. In order to do so I know I need to add a line of code to the theme’s index.php file just before the start of the Loop.

My problem is I can’t locate the index.php file. When I go to Appearance > Editor I see functions.php, customize.php, page_landing.php and style.css.

How do I access the index.php file and the Loop?

Thanks!!