Modify jpg DPI info without recompress?

I have JPG images produced by a camera that malfunctioned and wrote bad
DPI info (eg. horizontal vertical), and I would like to be able to change
the DPI info in the JPGs without recompressing. I would write a program to
do it, but I don't know how to locate the DPI info in the JPG files. Can anyone
point me to a free program that does this, or to a format description of JPG
files so that I can write one myself? If I do write one, I'll be happy to make it
available for free download, if anyone is interested.

Thanks!
--
Judson McClendon (remove zero)
Sun Valley Systems http://sunvaley.com
"For God so loved the world that He gave His only begotten Son, that
whoever believes in Him should not perish but have everlasting life."