Can't identify it, but don't prune it unless you really need to as you will probably cut off next years flower buds. Just dead head it in the spring. You can root semi ripe cuttings in late summer/early autumn, and they may root in water. Otherwise set them round the edge of a pot of compost, keep it out of strong sun and don't let it dry out. Try both ways.

It looks as if it could be H paniculata, try googling it,
if so you can prune this years flowering shoots by half in february or march.
you can take cuttings of 4-6" cuttings of strong non flowering shoots in august or september.