diff --git a/core/modules/field/field.crud.inc b/core/modules/field/field.crud.inc
index f6ee8a2..6c498be 100644
--- a/core/modules/field/field.crud.inc
+++ b/core/modules/field/field.crud.inc
@@ -533,12 +533,21 @@ function field_create_instance(&$instance) {
  *   - entity_type: The type of the entity the field is attached to.
  *   - bundle: The bundle this field belongs to.
  *   - field_name: The name of an existing field.
- *   Read-only ID properties are assigned automatically. Any other properties
- *   properties specified in $instance overwrite the existing values for
- *   the instance.
+ *   Read-only ID properties are assigned automatically. Any other properties
+ *   specified or omitted in $instance overwrite the existing values for the 
+ *   instance of the field. To preserve the existing field properties and make 
+ *   specific changes, first load the field definition with field_info_instance():
+ *   @code
+ *     $instance_info = field_info_instance($entity_type, $field_name, $bundle_name);
+ *     // Change a single property
+ *     $instance_info['definition']['required'] = true;
+ *     // Write the change back
+ *     field_update_instance($instance_info['definition']);
+ *   @endcode
  *
  * @throws Drupal\field\FieldException
  *
+ * @see field_info_instance()
  * @see field_create_instance()
  */
 function field_update_instance($instance) {