I have a white washable hat (somewhat like Gilligan's) that has turned

[raccoon]

yellow with age.

How to whiten it again?

[TygrBright]

1. First, pre-soak in HOT water plus a little bleach.

Second, wash in HOT water with bleach.

Third, rinse in HOT water with vinegar added.

Fourth, repeat, but this time use BAKING SODA in the rinse.

Then leave it out in the sun for a couple of days.

Might help.

speculatively,
Bright

[supernova]

2. Soak it in Biz

Biz is an enzymatic bleach.

Put it in a sink/bucket/pail with some water and I don't know, an ounce or so of Biz and let it soak for a few days. It will turn white again but it will take a while.