You can cut down the branches as you go, as this forces new shoots from the base. It takes time, but they will develop shoots, much like a bamboo forest.  But if you shouldn't cut all the branches down until the end of the season if you want it to remain a bush. This forces the trunk to harden (you are artificially dwarfing it, like you do with a bonsai) and many new shoots will spring up next season. If you want it to be a tree, leave the central branch, and let it develop smaller branches off the knots.