Most likely, yellow leaves on your Begonia are caused by watering the plant too frequently, allowing the soil to stay soggy, or letting the plant sit in water. This is an easy fix, going forward. Be sure to only water when the soil volume is 50-75% dry. Water thoroughly until you see water flow out of the drainage hole. Discard any excess water in the tray after a few minutes. If the soil is kept too wet for too long, the roots may rot.
Make sure that the plant is getting enough light. Keeping your plant in a spot with good lighting will help keep the soil from staying too wet. If your Begonia is in a lower light area, it will take longer to use water and can lead to overwatering symptoms such as yellow leaves. Moving your Begonia closer to a window can help improve the health of your plant.