You should cut off the top of the lily stem to remove the seed pods that are forming there after the flowers finish. The plant pumps a lot of energy into those seeds while we’d rather that energy be sent to the bulb below to be stored for the next flowering cycle and to create new baby bulbs below the soil. The remaining leaves should be left to gather sunlight (food) as long as they are green. Once they turn yellow they can be cut off. The longer you can maintain those leaves, the faster your lily bulbs will reproduce.