jng to jpg conversion is actually export between two variants of JPEG format, from the 'network' variant to standard JPEG. It will be hard to find a graphics editor that can still open old .jng file, but according to some sources, these files can open these files if you rename them as .jpg files, because the header might be same.
Except for that, you have to rely on some dedicated graphics converter that still supports JNG format.
Updated: October 16, 2023