I would also like to know.
In Settings > XML-Sitemap there is a checkbox:
Add sitemap URL to the virtual robots.txt file.
The virtual robots.txt generated by WordPress is used. A real robots.txt file must NOT exist in the blog directory!
If it’s virtual how do you edit configs you want to make?
FYI There’s no sign of any file anywhere which I’m not too surprised about if it’s virtual.
Also FYI My site is in a directory (not root) if that’s any use and I don’t have an existing robots.txt file.
cheers
Dr M