Click on "Hmwk Sets Editor" and under "Edit Assigned Users" click on the numbers (35/37 or whatever) to edit student information. Then click on "Edit data for" the student.
For the particular problem enter 1 for the "Status". This gives the student 100% of whatever the problem is worth. Actually "Fractional Correct" would be a better name than "Status". Then "Save Changes" and that's it. If you want to check what you did, look under "Student Progress".