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 within the Clojure store, enhancing maintainability. While the work addressed two targeted features rather than bug fixes, it demonstrated a methodical approach to incremental improvement and maintainable engineering within a focused project scope.