Your Dieffenbachia 'Tropic Snow' has done well under your care. Dieffenbachia is a kind of Jack-and-the-Beanstalk plant. It grows rapidly and ever taller. They often start to topple over under the weight of their large leaves. Eventually, they all have to be pruned back, as you surmised.
Pruning is easy. Cut the stem back to any height and that is where new growth will start to emerge and continue to grow upward from there. I recommend that you prune it back to a height of about a foot above the soil. That will eliminate most of the bare stem and leave lots of room for it to continue to grow upward before you have to prune it again. Use large sharp scissors or a sharp knife to make a clean cut.
The top section can be propagated. Remove all but 6-8 inches of bare stem below the lowest leaves. That top cutting can be rooted initially in water or in soil either in its own small pot or in the base of the existing pot. I don't recommend using the empty pot nearby as it is too large and makes proper watering very challenging. Rather, start it in a terra cotta pot that is about 6 or 8 inches in diameter. As it grows it can later be moved to a larger pot.
