It needs no pruning :-) Those branches coming out at an angle on top will eventually start to cascade as they elongate. FWIW, that umbrella shape in these kinds of JM's is not very uniform unless intentionally made to look that way. It is not my preference to force that sort of growth habit - I prefer a much more natural look - but that is your choice.

If you feel you DO need to prune, wait until December or in late June. These are the two best times to prune Japanese maples without excessive bleeding.

This is great advice. Many thanks!