How to rerun hook_post_update_NAME()
"post_update" hooks that have been run are stored in the database, in the key_value table, post_update collection, but the data is serialized and awkward to update directly.

I used some of the details from @kiamlaluno's answer to create a drush script which you can use to reset a single hook. Here's a basic version (longer version is here):

#!/usr/bin/env drush

$key_value = \Drupal::keyValue('post_update');
$update_list = $key_value-&gt;get('existing_updates');

$choice = drush_choice($update_list, dt('Which post_update hook do you want to reset?'));

if ($choice) {
  $removed_el = $update_list[$choice];
  unset($update_list[$choice]);
  $key_value-&gt;set('existing_updates', $update_list);
  drush_print("$removed_el was reset");
} else {
  drush_print("Reset was cancelled");
}


And here's an example of what it looks like when you run it from the command line:

./scripts/reset_hook_post_update_NAME.drush

Which post_update hook do you want to reset?
 [0]   :  Cancel
 [1]   :  system_post_update_add_region_to_entity_displays
 [2]   :  system_post_update_hashes_clear_cache
 [3]   :  system_post_update_recalculate_configuration_entity_dependencies
 [4]   :  system_post_update_timestamp_plugins
 [5]   :  my_module_post_update_example_hook

# The script pauses for user input. 
5 

my_module_post_update_example_hook was reset