You can update a student's password to help them regain access to their account. Click on the Students tab on your dashboard, find the student who needs your help, click on the three dots icon on the right and choose 'Change password'.
For data security reasons teachers must update their details themselves in their profiles, or use the 'Forgotten password' screen if they can't log in. For the same reason, email addresses can only be updated by the students themselves in their profiles.