Red Betta fish turning white

I think a little more information will help everyone give you better advice. There are a number of possible issues here. Does the fish have white cotton looking fuzz on its scales? If so that's fungus, and it can be treated with a variety of fish medications that you can find at most fish stores. Look for something containing "Ketoconazole" on the label if you think it is fungus. You might also try adding a little salt to your fish tank; changing the water conditions can sometimes kill external fish parasites and pathogens.

If it's NOT fungus, it could be a dietary problem- what's the fish eating? Maybe you want to consider a better brand of fish food or even some live food. Also, what's your lighting situation like? I've heard goldfish turn white if you keep them in the dark, I sort of doubt it but is that the case here?

Last but not least, consider changing about half the water in your aquarium. Draw some cold water from the tap and let it stand overnight to dechlorinate and come up to the same temperature as your tank. (Unless you have a heater, in which case you want some warm water that's the same temperature as the tank. I leave this up to you!) You'll want to do that about every other week going forward if you're not doing that already.