Add following code in your theme’s (child theme’s if any) functions.php
add_filter( 'fep_form_submit_button', function( $button, $where ){
if( 'reply' == $where ){
$button = '<button type="submit" class="fep-button" name="fep_action" value="'. esc_attr( $where ) .'">SEND</button>';
}
return $button;
}, 10, 2 );