How do I stop my white cloth mousepad from turning yellow?

You can’t stop it forever. UV light breaks down the fibers and optical brighteners, so sunlight through a window will yellow it in months. Keep it out of direct sun or store it in a drawer to slow it down. UV-blocking sprays exist but change the surface feel and won’t stop it entirely. If it bothers you, just buy a new one—or get a darker color next time.