What should I do if the plastic enclosure turns yellow?

You can use toothpaste and salt for cleaning. First brush the plastic enclosure with toothpaste like brushing your teeth, and then sprinkle with salt for scrubbing. After the yellow color fades, you can clean it. Or use toothpaste, baking soda, white vinegar and appropriate amount of water to make a mixture. Then put the yellowed plastic enclosure in the solution and soak it for about 15 to 20 minutes, and then scrub it.