How in the world do I fix my top blank screen on Nintendo dsi xl?

You either have a loose connection to your top screen or the ribbon cable to the top screen is damaged. Either way you will need a tri-wing screw driver to open it up to check. If it's a loose connection, that is easy enough to fix. If you need to replace the entire top screen with the ribbon cable, then there are tutorials on YouTube that can show you the process. Parts are easy to find eBay. Good luck!