you can either cut stems way down, just a few nodes above substrate. That way the new growth it sends out will cover the original stuff quickly. But the best option to make it look the best is to cut all the nice tops, set them aside and pull up all the originals. Then replant the nice new growth. I do a combination of those methods. nice new stuff in front and super trimmed originals in the back, so they are hidden by the nice stuff!
And speaking of trimming, I need to cut about %75 of my stems out too! Super overgrown...