UPDATE scores 
SET
    score = CASE 
        WHEN score < 95 
            THEN score + 5 
        else 100 
        END 
FROM
    students 
WHERE
    scores.student_code = students.student_code -- WHERE句に結合条件を指定
    AND students.student_name = '佐藤 幸平' 
    AND students.student_name = '山田 康介';