Since your dumb cane has grown tall and leggy, you can top the plant, or cut it back anywhere on the stem. Anywhere there is a node, you can grow roots in the right substrate.
To root dieffenbachia in water, cut stem sections that are 4 to 6 inches long and remove all the leaves. Place the cuttings in a glass of water, and change the water as often as needed to keep it clean. Keep the jar in a warm, brightly lit area out of direct sunlight. I use only RO or distilled water and change the water every 5 to 7 days,
The soil way to propagate your dieffenbachia is by rooting cuttings, either tip cuttings or stem cuttings. Place the small pieces or cuttings in a good medium for rooting and they will produce roots and, eventually, an entirely new plant. Cut the tips from the end of the plant or look for shoots coming from the main stem for the best plants..