Of course, if your N900 has problems with the display or the battery, you should just send it to warranty. If you're out of warranty, you should not try to repair it by itself. I've tried this with one of my smartphone, and I messed it really bad. So, you should go with a professional service.
If you have the courage to disassembly Nokia Lumia 900 by yourself, you should follow the steps in this video. If you plan to replace the display, you should check for a new one on Amazon or Ebay. Remember to read the reviews of the user you're buying from. If you need spare parts, you can check Nokia Lumia 720 Displayschutzfolie