Add a new column to the backend I am trying to add a new column to a table and I added the column using hook_update_N().
function cca_update_8902(&amp;$sandbox) {
  db_add_field('cca_findings', 'cca_topic_id', array(
    'type' =&gt; 'float',
      'not null' =&gt; FALSE,
      'description' =&gt; 'interim fields addition',
    ));
  \Drupal::entityDefinitionUpdateManager()-&gt;applyUpdates();
}

My field is also defined in the cca_findings class as property.
class cca_findings extends ContentEntityBase {

  use EntityChangedTrait;
    
  /**
  * {@inheritdoc}
  */
  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
    $fields = parent::baseFieldDefinitions($entity_type);
    
    $fields['cca_topic_name'] = BaseFieldDefinition::create('string_long')
      -&gt;setSettings(['max_length' =&gt; 2047]);
    $fields['meanvalue'] = BaseFieldDefinition::create('float')
      -&gt;setLabel(t('meanvalue'));

    $fields['cca_topic_id'] = BaseFieldDefinition::create('float')
      -&gt;setLabel(t('cca_topic_id'));

    return $fields;
  }

}

So, the column has been created correctly, but the values are not going on the backend. I feel I am missing something.
What is missing in the code I wrote?
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.