limiting postgreSQL update command
===

I have a table with given structure, now I want to write a query that will transfer 2 xx product from status 1 to say status 2. the child code is irrelevant presently to me.


    master_code| child_code | status_code
    -----------|------------|------------
        xx     |    xx1     |     1
        xx     |    xx2     |     1
        xx     |    xx3     |     1
        xx     |    xx4     |     2
        xx     |    xx5     |     2
        yy     |    yy1     |     3
        yy     |    yy2     |     2
        zz     |    zz1     |     1
        zz     |    zz2     |     1

I have implemented basic checks, and when I used 

    update only product_child
    set product_status=1
    where product_status=2

all three xx's got code 2, I want control over that, I was expecting only one xx will get code change with this command