Update and insert only specific fields of your entity

TO not need to use hook_entity_presave and save whole entity try this

//Get the id of your field
$field_name = 'name_of_your_field';
$info = field_info_field($field_name);
$fields = array($info['id']);

//Execute the storage function
field_sql_storage_field_storage_write($entity_type, $entity, 'update', $fields);