Is it dead wood? Are there any signs of new shoots at all? Do the twigs snap or bend, and have you tried a scratch test on the upper part? To do this, scrape a little piece of bark back and if the wood underneath is brown, and the twigs snap, then that part of your Acer is dead.

The only solution in that case is to cut off all the dead wood and try to make a visually pleasing shape. If there are any signs of life, then you'll have to leave it as it is and hope it regenerates.

if it is dead and you do chop out the top growth as spritz sais dont worry as the plant will soon grow into a better shape bye nature as long as there isnt a deeper problem causing the die back in the long run .