Sometimes, as plants get older, they start to die.  However, it may be possible that your Diffenbachia needs fresh soil and some fertilizer and more light.

Be sure to purchase potting soil, not garden soil or topsoil.

The container should have drainage holes and shouldn't be too big for the root system.  You may be able to use the same container.  Look at the root system when you remove it from the original container.  If there are more roots than soil, choose a larger container, about an inch or so larger. 

When watering, the soil should feel like a squeezed out sponge.  Don't let the container sit in a saucer of water, the soil needs to slightly dry out between waterings. 

You might have to experiment a little and hopefully these tips will be helpful.

Thank you for your question.