1) All dead stems should be removed from hydrangeas every year.
2) After the plants are at least 5 years old, about 1/3 of the older (living) stems can be removed down to the ground each summer. This will revitalize the plant.
3) In addition, if it becomes necessary to prune a plant to reduce its size, it may be cut back in June or July without harming the next year's bloom. But it will return almost immediately to it's former size. This is one reason why it's best to plant a hydrangea where it does not have to be pruned.