You can set a new due date for an assignment or assessment for a single student without adjusting it for everyone in the course. Here's how:
1. Click "Manage" and select "Assessments".
2. Find the assignment you want to adjust and click the "Submissions" icon to the right, under "Actions".
3. Find the student and click on the submission in question.
4. Click the "Actions" menu in the top-right corner of the submission section, and select "Reset Submission"
5. Set the new due date, and click "Submit".