Remove the flowered stems and leave the centre open and the new growth will soon appear.

but prune immediately after flowering as if you prune later in the year you won't get any flowers next year

Thanks , guys .
This bush that I want to prune has flowers all along each delicate branch , I don't think that I should be cutting the branches , should I ?
It will be a fiddle ( one that I will do) if I have to cut each small dead flower-head .
Still a bit at sea .

A photo might help. But if it is anything like my Philadelphus microphyllus occidentalis all the branches which have flowering spurs on them will be removed back to a couple of inches from the point of origin. New growth will then shoot up from these places for next years flowers. Wish I could add a photo to show you what I mean.

Hi , O'boggy .
I'll try a photo tomorrow .