Click on "Hmwk Sets Editor" link. Then click in the "edit assigned users" column in the row corresponding to the problem set you are interested in.
On the "users assigned to set ...." page you will see a list of student who have been assigned this problem set. Click on the link "edit data for studentA" where studentA is the login ID for the student whose problem you would like to change.
On the "set detail" page you now see the the details of studentA's assignment. You can change the seed for any one of the problems by entering a new number for the seed and clicking the "Save Changes" button. You can use any number for the seed.
This will change the appearance of the problem for studentA, but nothing else.