What lat9 says is correct.

You need to make a override version of that file. ( Or it is better to anyway )

To do that copy the file:

includes/templates/template_default/common/tpl_main_page.php

to

includes/templates/your_template/common/tpl_main_page.php

If this is causing the site to crash then you are doing something wrong. I know that sounds unhelpful but it is true, unfortunately. Or you are altering the file when you are copying it in some way.