There are a few different reasons that your poinsettia may develop yellow leaves. The most common reason for yellow leaves on a poinsettia is when the plant is getting either too much water or too little water. Make sure that you are watering your poinsettia when the top inch of soil in its pot is dry, but not before. When you give the plant water, keep watering until the moisture drips from the drainage holes at the bottom of the container.
Poinsettias can also develop yellow leaves as a result of a nutritional deficiency. When poinsettia plants have a shortage of magnesium or molybdenum, their leaves can turn yellow. However, over fertilization can result in yellow leaves as well.
Finally, root rot can result in yellowed foliage on a poinsettia. To check for root rot, gently pull up your plant to examine its root system. Plants suffering from root rot will have roots that are dark instead of the pale color of healthy roots, or the roots may be mushy and soft or slimy. If you see these signs, let the plant dry out on a stack of newspaper before placing it back in its container with fresh new soil. Make sure the container has drainage holes and that the holes are large enough to provide sufficient drainage. Consider switching soil to something that offers more air circulation and drainage. Finally, adjust your watering to avoid oversaturating the soil where your poinsettia is growing to avoid root rot problems.