A 5k race doesn’t last very long, so there is no need to carboload. I would suggest eating a full meal the night before. Maybe pasta, bread, salad, and fruit for dessert. The morning of the race eat a light breakfast of oatmeal or another cereal and some fruit. Do not over-hydrate yourself. Have something like Gatorade. If you are racing later in the afternoon, have some rice and fish for lunch and maybe another piece of fruit.