Correct way to append value to serialized array in key_value table
The best way to go about doing this is by using config API. If somehow this option is not suitable for your use case. You can also use the DB API to update the desired congratulation table records. In both cases, you will need to unserialize the blob then change the data and serialize it back before inserting it into the database.

https://www.drupal.org/docs/8/api/configuration-api/configuration-api-overview