Just enter the password you want this user to have into each text field on this page and click the "Change Password" button. Entering different text in the two fields will result in the password not being changed, as will leaving one or both fields blank.
If you got here by adding a new user, bear in mind that the user has already been created with a password of "change". If you cancel out of this screen, or don't change the password, the user will still exist with the default password ("change"). You might want to delete them at this point if you didn't want to create this user after all.