As a teacher, you can update students accounts to update their password if it is lost or forgotten.   You are not able to update the username as it is a unique field used to identify the student.   To update a student account, you need to be logged in and do the following:

1. You must be logged in as a teacher
2. Click My Dashboard 
3. Click on My Students
4. Click the tab labeled All Students
5. Find the student you wish to change the password for, and click on the button Edit Student
6. A popup will display the students information including the the password.   Enter the new password in both the Password and Confirm Password fields
7. Click Save