It depends on what type of hydrangeas you have. First find out and if in doubt, you can probably just not prune them and they'll be fine.
Prune smooth hydrangeas (the ones with white, fluffy, rounded white flowers, such as 'Annabelle') in late winter or early spring, cutting back to one quarter their original height. Or you can also just leave it alone and it will still bloom quite well.Prune bigleaf (mophead) hydrangeas, the ones that are pink or blue or are the lace-cap type, only by removing the flower heads after they fade.And if you have a peegee hydrangea, the type with white more cone-shaped flower heads, avoid pruning. It does best without.