How did you get the books into Stanza in the first place? If you purchased books through one of the stores inside of Stanza, you should be able to re-download any purchases by visiting whatever store in your browser on your PC (but be aware that those purchases will be DRMed and won't be able to be put on your Kindle). If you got them from free sources like Feedbooks or Manybooks or whatever, just go download the books again from the website.
The only other option I can think of is to jailbreak your iOS device, find the books in the filesystem, and copy them off. I would really suggest not going this route if you can avoid it. Your best bet is to figure out where you got your books in the first place and get them from there again.