Cactus

You have two choices.  Find the plant a new home with a higher ceiling or prune it back.  Both are difficult choices.  Wait until spring to prune back your plant.  Cut the stem back to the place where you would like new growth to begin.  It will take several days even a week for the cut to seal.  New growth will eventually sprout from beneath the cut.  Discard or use the top to start new plants. Cut the top into smaller, 4 to 6, inch pieces.  Let them dry for a couple days until the cut ends seal. Always plant the cut end that was closest to the roots, down. Fill a pot or flat with perlite, vermiculite or a well-drained potting mix.  Set the cuttings on, not in, the mix to avoid rot.  Try supporting the cuttings with a thin layer of gravel sprinkled over the rooting media surface and around the stem.  Or try using a small bottomless pot (set over the cutting and sunk into the rooting media), stake or other method to support the cuttings.   Plant rooted cuttings in a well-drained growing mix.