How to Update Configuration Entity in Drupal 9+?

In Drupal 9+, updating the existing Configuration Entity is a bit different than in Drupal 8. Drupal 8 has drush entity:updates, drush entup, drush entity-updates the command to update the existing entity schema but Drupal 9+ doesn't have these commands. See the overview of Configuration API.

How to update Configuration Entity in Drupal 9+?

Steps to update Configuration Entity in Drupal 9+:

1. Suppose you have custom_module with project_config_entity Configuration Entity.

2. You want to add two new fields configuration and json_configuration in the custom Project Config Entity.

3. Default configuration when generating by drush generate configuration-entity of Project Config Entity

File: custom_module/config/schema/custom_module.schema.yml

custom_module.project_config_entity.*:
  type: config_entity
  label: Project Configuration Entity
  mapping:
    id:
      type: string
      label: ID
    label:
      type: label
      label: Label
    uuid:
      type: string
    description:
      type: string

4. Add new fields in the custom_module/config/schema/custom_module.schema.yml file as below:

custom_module.project_config_entity.*:
  type: config_entity
  label: Project Configuration Entity
  mapping:
    id:
      type: string
      label: ID
    label:
      type: label
      label: Label
    uuid:
      type: string
    description:
      type: string
    configuration:
      type: string
    json_configuration:
      type: string

5. Add fields in the custom_module/src/Entity/ProjectConfigEntity.php file under the config_export key as below:

 *   config_export = {
 *     "id",
 *     "label",
 *     "description",
 *     "configuration",
 *     "json_configuration"
 *   }

6. Add new fields in the class ProjectConfigEntityForm of custom_module/src/Form/ProjectConfigEntityForm.php file as below:

    $form['configuration'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Configuration'),
      '#default_value' => $this->entity->getConfiguration(),
      '#description' => $this->t('Configuration in text format.'),
    ];

    $form['json_configuration'] = [
      '#type' => 'textarea',
      '#title' => $this->t('JSON Configuration'),
      '#default_value' => $this->entity->getJsonConfiguration(),
      '#description' => $this->t('Configuration in the JSON format.'),
    ];

7. Declare methods in the class ProjectConfigEntityInterface of custom_module/src/ProjectConfigEntityInterface.php file as below:

namespace Drupal\custom_module;

use Drupal\Core\Config\Entity\ConfigEntityInterface;

/**
 * Provides an interface defining a project configuration entity type.
 */
interface ProjectConfigEntityInterface extends ConfigEntityInterface {

  /**
   * Declare function to get the configuration.
   */
  public function getConfiguration();

  /**
   * Declare function to get JSON configuration.
   */
  public function getJsonConfiguration();
}

8. Add members variables & methods in the class ProjectConfigEntity of custom_module/src/Entity/ProjectConfigEntity.php file as below:

  /**
   * The project_config_entity configuration field.
   *
   * @var string
   */
  protected $configuration;

  /**
   * The project_config_entity json_configuration field.
   *
   * @var string
   */
  protected $json_configuration;

  /**
   * Get Configuration value.
   *
   * @return string
   */
  public function getConfiguration() {
    return $this->configuration;
  }

  /**
   * Get Configuration in JSON format.
   *
   * @return JSON
   */
  public function getJsonConfiguration() {
    return $this->json_configuration;
  }

9. Do drush cr and open the Form /admin/structure/project_config_entity/add and see the new fields.

10: Verify your changes in the Edit mode as below: