Adding / Updating custom entities to an existing module

When a module is already enabled, you have to use the Entity Update API to make updates that involve schema changes.

Automatic entity updates are now removed, in this post are some useful samples of update hooks to different cases:

Support for automatic entity updates has been removed

For installing a new Entity, the following code can be used to avoid duplicating the entire definition in an update hook:

/**
 * Installs the new "__your_entity__" entity.
 */
function your_module_update_9001() {
  $db = \Drupal::database();
  if (!$db->schema()->tableExists("__your_entity__")) {
    \Drupal::entityTypeManager()->clearCachedDefinitions();
    \Drupal::entityDefinitionUpdateManager()->installEntityType(
      \Drupal::entityTypeManager()->getDefinition("__your_entity__")
    );
  }
}

Related

This link provide initial sample on creating a custom entity:

Creating a custom content entity

Updating Entities and Fields in Drupal 8

Updating Entities and Fields in Drupal 8