It may become necessary to repot your Cereus if it outgrows its container. If so, make sure the soil is dry and then remove the pot. Knock away old soil and prune away rotted or dead roots. Replant in a new pot and backfill with fresh soil. Make sure not to overwater as this can lead to root rot.
These cacti propagate easily from cuttings. Simply cut a branch and replant in moist, well-drained soil. The branch should be left to dry for about a week before potting and then watered lightly.