Lower leaves can turn yellow when the plant has been overwatered. Double-check the plant is not sitting in water and if it is, pour this away. Next, remove the yellow leaves and only give your plant some water every 3-4 weeks. Alternatively, check it to see if it feels light in weight or if the soil looks dry - those are signs that it now needs a water. Sometimes the top layer of the soil can appear dry but when you pick up the plant there is some weight to it, as the soil is moist underneath, so it is good to do both of these checks.