There is no linux program I can find that can change the jpeg header so
that image remains the same but ppi information is changed. Sometimes
one is requered to provide an image with a minimum of 300dpi.

I see jpegtran a natural fit for that since it does various
manipulations of the jpeg headers already.

One option is to modify exif headers but then not all programs regard to
them. Another option is to use gimp but then you either have to transfer large
files or lose quality by recompressing the resulting image back to jpeg.

I can't find any information how to contact jpegtran upstream maintainer
(if any).

Regards,
Aleksandar

Hello, sorry for the delay. I forwarded it to upstream but forget to
send you the answer.

Technically this belongs to the JFIF header not the JPEG header.

Upstream suggests to add it to wrjpgcom.

Sure there is, see /usr/share/doc/libjpeg8/README.gz

Cheers,