If the assignment availability and cutoff dates have already passed, but you have or more students that need an extension you can do the
following:

1) Change the availability dates within the assignment
a. Select the Action Link and choose Edit Properties
b. Within the Assignment Properties, edit the Dates section to reflect the new due date and cutoff dates for the extension
c. save the changes

2) Set the Release Criteria for the assignment to add a "Member Criteria" and set the Member Criteria to be the NSID of the student who needs the extension
a. again, use the Action Link, choose Set Release Criteria
b. choose the Add Member Criteria option
c. Select the User ID of student(s) needing an extension using the checkbox
d. Save the changes