do nothing until late spring then you just need to remove the dead flower heads. if you want to remove some of the straggly side stems then you can but the buds lower down the stems wont flower that year. not all of them get any real height mine about 3ft and that's all, others 4ft.

Thanks SBG I'll tidy it in Spring.

Hydraneas are one of the most forgiving bushes you can have. It will always come back. That being said, what type of hydrangea do you have? Some tend to sprawl, others go for height.