$arFilter = array(
	'IBLOCK_ID' => 4,
);

$arSelect = array(
	'ID',
	'CODE',
	'IBLOCK_ID',
	'NAME',
	'UF_CHOOSE_RULE'
);

$dbSection = CIBlockSection::GetList(array(), $arFilter, false, $arSelect);
while( $arSection = $dbSection-> GetNext() ){
   $sections[] = $arSection;
}

//print_r($sections);

$bs = new CIBlockSection;
foreach ($sections as $k => $sec) {
	$bs->Update($sec['ID'], array('UF_CHOOSE_RULE' => '6'));
}