Henri Lehtinen contributed to the Opetushallitus/ataru repository by simplifying the data submission model in the tutu application, specifically removing the country field to streamline the submission process and reduce unnecessary complexity. He focused on backend development and database management, utilizing Clojure and SQL to implement these changes. Henri also improved codebase clarity by renaming a SQL query for better descriptiveness and updating the corresponding function call in the Clojure store, enhancing maintainability. Over the course of the month, his work addressed targeted feature development rather than bug fixes, demonstrating a methodical approach to incremental backend and data model improvements.