Also depends on what sort you have, whether mophead or paniculata as they have different pruning requirements. More info can be found here https://www.rhs.org.uk/advice/profile?pid=516
Why is it flopping over like that? Is it too shaded or has it been pushed over by another plant? I would prune it down to 1-2ft (possibly in early spring rather than now) and let it come back and establish a better shape, after moving it to a better site. You'll lose one season's flowers but hopefully end up with a better shaped shrub.
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Do not prune.
More info can be found here
https://www.rhs.org.uk/advice/profile?pid=516