Topping a tree usually has disappointing, bordering on disastrous results. The crown becomes ragged without a central leader. You are then forced to keep pruning to maintain a pyramidal shape. These trees are sometimes grown as bonsai so pruning is possible. Instead of looking to change the tree, first consider whether your site conditions meet its needs. Japanese umbrella tree requires acidic, rich soil, prefers full sun (6 hours daily) with protection from hot afternoon sun and consistently moist soil. The last condition is tricky because the tree does not let wet or clayey soil. It also needs protection from winter winds which strip moisture from the "needles" and might kill an emerging branch. If you aren't meeting its needs, it will not perform well. In zone 5, you are on the edge of its hardiness range. It may be set back if your winters have been more harsh than typical. Images on the internet show some highly sculpted trees but I found no information on pruning to encourage more branches. I did read that insufficient sunlight may result in a leggy, open appearance. good luck
Never top a tree, you can remove bottom branches if needed.
Review the care needed in the article below.