It matters what kind of hydrangeas they are- if they are macrophyllas that bloom on old wood leave them alone or else you risk losing next year's flowers. You'd prune these next year immediately after blooming has finished.

If they are paniculatas you can remove any unsightly old blooms- if they are dry enough you can just run your closed hand up each stem and they fall right off.

I wouldn't actually prune anything this late in the season as it triggers new growth that is easily damaged by freezes.

I know the ones I have bought were Bloomstruck and Quick Fire, the existing ones I am not sure of but the plants are quite large and produce large blooms which are now just brown and still on the plant

seeing pictures I believe the existing Hydrangeas are macrophyllas

Bloomstruck - most likely, it & other macrophyllas may have already started to develop invisible flower buds at the ends of the stems. You do not say where are you but their invisible flower buds may open in Spring 2019 if winter protected so do not prune the stems now as cearbhaill suggested until after blooming has ceased next year.

Quickfire - paniculatas develop invisible flower buds late in the growing season (June-ish). You can prune now if you dislike the pink or brown blooms. Or leave them alone for winter interest and prune them in Spring.

This link may help you identify the types of hydrangeas, which is what we need to know: https://plantaddicts.com/types-of-hydrangeas

If you have mostly macs then my suggestion is to leave any pruning to a bare minimum. You can deadhead whenever you want but deadheading is not the same as pruning. And pruning macs at just about any time is likely to remove potential flowers.

It's all well and good to say that you can prune safely when flowering stops. But the flowering of macs, especially reblooming macs, doesn't have a defined cut-off date. In my climate they will continue to bloom well into fall as long as the weather is agreeable. When they finally do stop, it is long after the latent flower buds have been set for the coming season, so far too late to prune!!

On macs and other old wood bloomers, I suggest not pruning at all except to remove any deadwood that fails to leaf out in spring. You can do rejuvenation pruning if the shrub gets too large for its site or becomes old and develops a gnarly, woody interior but be prepared to eliminate or reduce flowering if you do.

And it is always preferable to prune any shrub in spring rather than fall, which can often expose plant tissue to unnecessary cold damage.