updateCustomField($fieldId, $fieldValues)
You can find the id of the field by going to Admin -> Settings -> Set up custom fields for -> Go and when you hover the field you are interested into you will see the id.
Getting the value is more tricky. What works on any type of field is setting a value manually and then querying DataFormField for the value like this:
$returnFields = array('DataType', 'Id', 'FormId', 'GroupId', 'Name', 'Label', 'DefaultValue', 'Values', 'ListRows');
$query = array('Id' => custom_field_id);
$res = $sdk->dsQuery("DataFormField", 10, 0, $query, $returnFields);
var_dump($res);
In the result above, have a look at the "Values" field, this is how it should look like.So to conclude, if you need to update a listbox custom field, this will do it:
$values = array(
'Values' => "\naaa\nbbb\nddd"
);
$result = $sdk->updateCustomField(custom_field_id, $values);