You can trim either the top or the bottom like mgamer says. I always pull the stem out and trim the bottom three or four inches and replant. You can trim off any length you want. I always leave the tops on since they have the prettiest leaf arrangements. Some people leave the stems in the substrate and trim off the tops. If you trim the tops the plant will grow two or three new stems from the cut-off top area, making the plant bushier and fuller. This would be a good method to make a plant fill in an area better.