Try this: in archive-header.php
line 8 – 10 by interecepting the variable “period=past” or ( I guess) “period = future” of current URL
<?php else :
if( !empty($_GET['period'] ) && $_GET['period'] == 'past')
{
echo "Past Events";
}
elseif( !empty($_GET['period'] ) && $_GET['period'] == 'future' /*or whatever*/){
echo "Upcoming Events";
}
else{
echo esc_html(lsvr_pressville_get_event_archive_title());//the default output of your theme function
}
endif; ?>