Open the door fully, push a suitably sized nail or other metal rod
through a link of the chain adjacent to the portion of the closer which
is in the door.
Remove the screws and withdraw the unit from the door.
The door part is threaded together and can be unscrewed to remove some
of the tension from the spring.
Refit the unit, push the door open to tension the spring, and remove
the nail.
Please note that, all the time this type of unit is fitted to a door,
the door will slam to a greater or lesser degree depending on the
tension of the spring.