Option 1
On the login screen, the student can select 'Forgot Password?' by the password field.
The student can then enter their email address, and they will receive an email with a link to reset their password.
Option 2
Once you've opened a class, go to the 'Students' sub-tab.
Select the 3 action dots next to the 'Edit' button for that student.
Choose 'Reset Password'.
You will then be able to copy an auto-generated temporary password to give to the student.
The next time the student logs in with the temporary password, they'll be prompted to create a new password.