Prunning a saguaro cactus
It is very difficult to prune large cacti, particularly columnar ones. Here are two websites which may help.
It may be wiser to take cutting from your cactus and start some new plants but even this is tricky. To do this, cut off one or more of the side shoots (about 6 inches long). Let the cut surface dry out for about three weeks then treat the cut end with rooting compound. Insert the cutting into a small pot containing cactus potting mix and place in a location with moderate light. Do not water for the first few months and then only sparingly. After a few more months pull gently on the plant to see of it has grown roots and then transfer to a larger pot if necessary. Take the cutting in spring or summer. Wear gloves when you are working.
Another way to propagate is to cut the main stem of the cactus down to about 2-3 ft. and let it regrow, You will probably get numerous shoots growing out of the side of the cut. When these are larger than 2 inches long you can cut them off and root them as described above.
Unfortunately, you may want to consider giving up on this plant and starting anew. Incidentally, from your description it sounds as if your present plant is very attenuated. Place your new plant in the sunniest possible spot as cacti need very strong light to grow well.
Courtesy of NYBG Plant Information