Cement board siding should be repaired by a professional. If you are not a professional, do not attempt to repair it.

Well, it depends on what's wrong with it. If the cement board is cracked or broken, you can use a patching compound to fix it. If the surface is damaged or stained, you can use a concrete cleaner to restore it. If the board is missing or loose, you can replace it with a new piece. And if the entire board needs to be replaced, you can purchase a new one at your local hardware store. Just make sure you follow the manufacturer's instructions for installation.