Yellow leaves are most often a sign of overwatering, poor drainage or some other form of root damage if it has happened quickly. Is the plant in a pot or in the ground? Yellow leaves can also be a sign of lack of  fertilizer but that would happen slowly over time. Too much heat or transplant shock will also turn hibiscus leaves yellow in a hurry. Are the yellow leaves just on the bottom of the plant or all over? First thing to do is check the soil by poking your finger down two inches . If it is moist do not water. Do not let a potted plant set in drainage water. Shade the plant if your afternoon sun is above 90 degrees. Look on the back of the leaves for signs of insects or webbing. You can write again with more details if these suggestions do not work. Keep growing, Marianne Binetti