Entire file of NEF to JPEG

You can batch convert RAW files to JPEG using Export panel in Adobe Bridge. Select files which you want to convert and drag them onto Custom Export preset in Window > Export panel. New dialogue will appear where you can choose settings. You can also save preset for later use, in that case drag files onto [custom preset].

As stated above- Not possible to "change"- only to create new JPGs from the raws.

If you have Lightroom-Classic:

Import all the NEFs,  Select them all in the Library, Export derivatives to JPGs.

Or this link may provide an answer- http://michaeltapesdesign.com/instant-jpeg-from-raw.html

Only if you can walk on water.  The Raw mosaic image  need to be converted into an RGB image.  How can that be done with opening the raw file to read the read the mosaic image data.  It like asking someone to translate a book from French to English without opening the book.

Do you have unlimited  machine resources where you can open all your NEF file at once and convert  all the images at once into a huge montage.

Use a Batch Processor like Image Processor Pro to do the job. It will do one file are a time and you can drink some wine while its doing its job.