Finding main loop code in Index.php?

Hi there!

I need to replace the standard/main loop code inside of the index.php file. It is supposed to look like this:
A. if ( have_posts() ):
while ( have_posts() ): the_post();

endwhile;
endif;

And I need to replace all this code with this new code:
B. The_Grid(‘My Grid Name’, true); // where true is for template mode

I have looked everywhere, and everywhere it states that the Main Loop Code (A) is inside every index.php file, irregardless which team you are using. I have searched all inside my Index.PHP file, and I cannot find Code A anywhere?! I need to replace it with stated Code B.

Please can someone advise me with HOW can I find my main loop code (as I can’t find it inside my Index.php file), so I can replace it with Code B?

Thank you, very much appreciated!!

Best,
Jennifer