Add a new column to the backend
As described in Support for automatic entity updates has been removed, when a module needs to update its own entity, and the update must change the database schema, the module needs to implement hook_post_update_NAME(). That is what the Taxonomy module does in taxonomy_post_update_make_taxonomy_term_revisionable() to make its entity revisionable.
The key code lines in that hook, which show how a new entity field is added, are the following ones.
$definition_update_manager = \Drupal::entityDefinitionUpdateManager();
$last_installed_schema_repository = \Drupal::service('entity.last_installed_schema.repository');
$entity_type = $definition_update_manager-&gt;getEntityType('taxonomy_term');
$field_storage_definitions = $last_installed_schema_repository-&gt;getLastInstalledFieldStorageDefinitions('taxonomy_term');
$field_storage_definitions['revision_id'] = BaseFieldDefinition::create('integer')
  -&gt;setName('revision_id')
  -&gt;setTargetEntityTypeId('taxonomy_term')
  -&gt;setTargetBundle(NULL)
  -&gt;setLabel(new TranslatableMarkup('Revision ID'))
  -&gt;setReadOnly(TRUE)
  -&gt;setSetting('unsigned', TRUE);
$field_storage_definitions['revision_default'] = BaseFieldDefinition::create('boolean')
  -&gt;setName('revision_default')
  -&gt;setTargetEntityTypeId('taxonomy_term')
  -&gt;setTargetBundle(NULL)
  -&gt;setLabel(new TranslatableMarkup('Default revision'))
  -&gt;setDescription(new TranslatableMarkup('A flag indicating whether this was a default revision when it was saved.'))
  -&gt;setStorageRequired(TRUE)
  -&gt;setInternal(TRUE)
  -&gt;setTranslatable(FALSE)
  -&gt;setRevisionable(TRUE);
$definition_update_manager-&gt;updateFieldableEntityType($entity_type, $field_storage_definitions, $sandbox);

In your case, the code for hook_post_update_NAME() would be similar to the following one.
$definition_update_manager = \Drupal::entityDefinitionUpdateManager();
$last_installed_schema_repository = \Drupal::service('entity.last_installed_schema.repository');
$entity_type = $definition_update_manager-&gt;getEntityType('entity_machine_name');
$field_storage_definitions = $last_installed_schema_repository-&gt;getLastInstalledFieldStorageDefinitions('entity_machine_name');
$field_storage_definitions['meanvalue'] = BaseFieldDefinition::create('float')
  -&gt;setLabel(t('meanvalue'));
$field_storage_definitions['cca_topic_id'] = BaseFieldDefinition::create('float')
  -&gt;setLabel(t('cca_topic_id'));
$definition_update_manager-&gt;updateFieldableEntityType($entity_type, $field_storage_definitions, $sandbox);

Replace 'entity_machine_name' with the machine name of the entity.