You can change the username and password by first going to the Web Terminal and clicking on the "Login As..." button. When you select a volunteer in the list, you will see their username and password appear at the bottom of the window. You may change the username or password by clicking on the little arrows that appear to their right.
Additionally, a volunteer may change their MSP generated password through the My Profile tab of the Web Terminal. If they change their own password, you will no longer be able to access their password (to protect their information, in case they use the same password for email or bank websites).
To learn more about password security, check out this article on MSP's blog!