AAcers will respond very well to pruning. It is best to prune whilst the tree is dormant, so December to February would be an ideal time of year.
Make sure to prune back to a bud – this means cutting just above the bud. If you leave any excess wood above the bud, the dieback could become diseased.
Simply prune the acer tree to the shape that you want it to be. As always with pruning trees, take out any dead, damaged or diseased wood, and any branches that cross over or are growing in the wrong direction.
If you have a smaller Japanese acer, sometimes the branches can get congested towards the centre of the tree. If so, you may also want to open it up to allow light and air movement through the tree. To do this, cut back a few of the older branches right back to the main trunk.