Not sure -- all depends how they've built their site.
Typically would be in any of:
/includes/languages/english/html_includes/classic/define_main_page.php
/includes/languages/english/html_includes/define_main_page.php
/includes/languages/english/classic/index.php
/includes/languages/english/index.php
or maybe even hard-coded wrongly in:
includes/templates/classic/templates/tpl_index_default.php
or
includes/templates/template_default/templates/tpl_index_default.php