How to fix "Mismatched entity and/or field definition" in Drupal.

I encountered this issue when upgrading from Drupal 8 to  

Mismatched entity and/or field definitions The following changes were detected in the entity type and field definitions. Paragraph The paragraph.field_background_fixed field needs to be updated.

Since <drush entup> is deprecated you would need to install the Devel Entity Updates for this. How I fixed this was to write a simple hook_update() function in my custom module:

/**
 * Drupal 9 upgrade: "Mismatched entity and/or field definition” when “drush entity-updates” fails?"
 */
function mymodule_update_9020() {
  $manager = \Drupal::entityDefinitionUpdateManager();
  if ($field = $manager->getFieldStorageDefinition('field_background_fixed', 'paragraph')) {
    $manager->updateFieldStorageDefinition($field);
  }
}

After the update, the error disappeared!

Background:

drush entity-updates (aka drush entup) has been deprecated as of Drupal 8.7.0. You either need to write an update script using hook_update_N(), or you can use the Devel Entity Updates module, which does the same thing as drush entup did previous to 8.7.0.

https://drupal.stackexchange.com/questions/198628/how-do-i-fix-a-mismatched-entity-and-or-field-definitions-error-on-my-configur