How can I update the spell checker on my computer?

I'm sure there's different versions of these things. When I use certain words it underlines them and doesn't have the correction I would need even if I WAS misspelling them..For example, ownage, ipod, smackdown, I can't remember them right now. If there's a way to update the spell check, and possibly get a grammar-checker I'd be very grateful to find how to get them, Thank you.