As a System Administrator you can change your Users' passwords whenever you like (as well as controlling how frequently a new password is required by the system).

To give a User a new password, open their record (Start > Configuration > Users & Security > Users) and pick their name from the grid on the right.

Open the reveal section titled Password and enter their new password twice.  (If you want them to change their password to one you don't know when they next login, tick the checkbox next to Change at next login.)

Click Save & Close and remember to let your User know their new password.