To enter user management menu go to My_site_name/wp-admin/users.php and you will see the following:
Press "Add New" button to add a new user. Fill out all of the required information (Username is mandatory to log in) and press "Add User"
To edit or delete user push "edit"/"delete" button near the username icon.
Here you may change account details and password. don't forget to press "Update user" button.