Tooth decay is the actual rotting of your teeth. The enamel on your teeth does not replicate so there's no way to reverse it if it's damaged.
However you can prevent tooth decay by brushing your teeth with fluoride toothpaste and chewing fluoride gum. Fluoride won't cure tooth decay (nothing will) but it will react with the calcium in the enamel and cause it to harden considerably. The hardening of your enamel makes it several times more difficult for bacteria to eat away at it...but it won't help against gum disease so brushing is still the ideal.