Question
What is the best way to take care of yellowing leaves and why are they turning yellow?

Answer
Dieffenbachias are sensitive to overwatering.  They need well-drained potting soil, watered only when reasonably dry.    Overwatering turns lower leaves Yellow and damages roots.  How much have you been watering this plant?  A little too much, maybe?

By the way, it is NORMAL for a HEALTHY Dieffenbachia to lose some older leaves as it gets bigger and grows larger.

But this can also indicate too much moisture LOSS is usually to blame.  This can happen with a Dieffenbachia that is kept in a hot room, at low humidity, for long periods of time, without enough water to draw.

Spider Mites also attack these plants, causing leaves to yellow and drop.  Check the undersides of the leaves for tiny mites, or shake a leaf over a white sheet of paper.

Let me know if this isn't clear.  Any more questions, I'm here.  Thanks for writing.