How do I update existing Nodes field through Import without having to delete all content and re-importing everything?
I'm pretty sure you'll be happy with the excellent Feeds module, which does this every day for thousands of sites. More advanced/customizable techniques are available with Migrate module, which always requires code, but gives enormous flexibility. Both will allow you to update existing nodes. Had you used them for your initial import, you'd be done now. I think you still may be able to use them as long as you have a unique identifier on your existing nodes.