A simple way to clean the yellowed surface is with an all-purpose cleaner that contains bleach. You can also wipe the area with a magic eraser, or for a more intensive cleaning, use a mixture of baking soda (1/4 cup), warm water (4 cups), and bleach (1/2 cup). When using bleach, make sure to wear gloves. Wipe down the surface with the solution, then wipe the area clean with a fresh rag or cloth. Repeat this as needed.