UPDATE scores AS sc
SET score =
    CASE
        WHEN sc.score + 5 <= 100 THEN sc.score + 5
        ELSE 100
    END 
FROM
    students st
WHERE
    sc.student_code = st.student_code 
    AND st.student_name = '佐藤 幸平' OR st.student_name = '山田 康介';