Which variety of Hydrangea? If its the ordinary mophead type, there no time to cut them back without losing next year's flowers...the best you can do is to remove some its stems right at the base, leaving some as they are. if its a paniculata, you can cut that back at the end of the season a/nd it will flower next year.

Just how big is your hydrangea? These shrubs grow 8 feet tall & wide. A better option is to transplant it to a more spacious location. Do this in the fall - partial shade. Otherwise you'll be hacking at it every year, cutting off next year's flowers and ruining it's natural form. I'm just guessing here, if there's a neighboring shrub you can move instead. That would give it more space.