add_filter('frm_custom_html', 'frm_customize_html', 20, 2);
function frm_customize_html($default_html, $field_type){
  //$default_html = 'change HTML here';
  return $default_html;
}

Move the field description above the field input. Note: This code snippet will only be applied to fields created after this snippet is added.

add_filter('frm_custom_html', 'frm_move_field_description', 20, 2);
function frm_move_field_description( $default_html, $field_type ) {
    $start_description = '[if description]';
    $end_description = '[/if description]';
    $description_start_pos = strpos( $default_html, $start_description );
    $description_end_pos = strpos( $default_html, $end_description );

    if ( $description_start_pos === false || $description_end_pos === false ) {
       return $default_html;
    }

    $description_length = $description_end_pos - $description_start_pos + strlen( $end_description );
    $description_string = substr( $default_html, $description_start_pos, $description_length );
    $default_html = str_replace( $description_string, '', $default_html );
	$default_html = str_replace( '/label>', '/label>' . $description_string, $default_html );
//  $default_html = str_replace( '<div class="frm_opt_container"', $description_string . '<div class="frm_opt_container"', $default_html ); uncomment this line if you have a radio/checkbox field in the form

    return $default_html;
}