Take it off, and put it in a bucket. Pour in a water-bleach mixture of about 10 parts water to 1 part bleach (It can go stronger if the staining is REALLY bad)
Let that soak overnight. It SHOULD bring it back to a brilliant clear white.
Drain the water/bleach mixture off. Let it dry completely, then soak it in clear water. Use some de-chlorinator. Keep rinsing/drying it until there is NO hint of bleach smell...
I tend to go to an extreme with this, and make sure they dry in direct sunlight, as direct sunlight deactivates bleach/chlorine as well...