Queries :: How To Reduce Number Of Item With Update

How do you update a table by reducing a number by 10?

My assignment question is:

10 students have left GY101. Write an SQL UPDATE statement to reduce the class size by 10 for all modules taken by GY101 students.

I can display the students who take GY101 with the following code

SELECT moduleCode, classSize
FROM ROOM_BOOKING1
WHERE moduleCode IN (SELECT modCode
FROM STUDENT_REG1
WHERE sID IN (SELECT id
FROM STUDENT
WHERE courseCode = "GY101"));

But how to reduce the number by 10.