How to edit part of the home page

I want to change the last 2 and cannot find where to go for that. I have gone to EDIT THEME>MAIN INDEX TEMPLATE file to change page it links to but it does not work.

change last two what clarify

Thanks for picking up the post!
I want to:
a) change the lower right page link (excerpt of Status of sharks?& rays) to an other post page. Something I will do regularly.

b)I have tried to replace the 3 photos with a video and it does not work

Please note that the 3 photos and text above are under HOME page

Will need to see the code but i think they must be coming from posts on left and on right is aboutus1 which must a page.

Here is the code. In the 2 instances where the code has =about-sharks-and-rays I have tried to change the name to the new post I want at its place. The text just disappears. Does it matter that I am trying to replace a page by a post?

</div>
<div class=”aboutus1″>
<?php $welcome = new WP_Query(‘pagename=about-sharks-and-rays’);
if($welcome->have_posts()) : while($welcome->have_posts()) : $welcome->the_post(); ?>
<h2><?php the_title();?></h2>
<?php get_the_content_limit(500, ‘read more…’); ?>
<?php endwhile; endif; ?>

</div>
<div style=”clear:both”></div>
</div>
</div>
<?php }else{?>
<div id=”content” class=”content1″>
<div class=”content2″>
<div>
<?php if (have_posts()) : while(have_posts()) : the_post(); ?>

<h2><?php the_title();?></h2>

<?php the_content(); ?>

<?php endwhile; endif; ?>
</div>
<?php if(!is_page(‘sponsors-and-partners’) )
{?>
<!–<div id=”homepage”>
<div class=”news”>
<h2>News</h2>
<?php $my_query1 = new WP_Query(‘category_name=news&showposts=3’); ?>
<?php while ($my_query1->have_posts()) : $my_query1->the_post(); ?>
<?php get_the_content_limit(40, ‘read more…’); ?>

<?php endwhile; ?>

</div>
<div class=”aboutus”>
<?php $welcome = new WP_Query(‘pagename=about-sharks-and-rays’);
if($welcome->have_posts()) : while($welcome->have_posts()) : $welcome->the_post(); ?>
<h2><?php the_title();?></h2>
<?php get_the_content_limit(400, ‘read more…’); ?>
<?php endwhile; endif; ?>

</div>
<div style=”clear:both”></div>
</div>–>
<?php }?>
</div>
<h2 class=”bottombg”></h2>
</div>
<?php }?>