How to change the CMYK values for entire document?

I have a label printer that prints very dark (I'm guessing the RIP sucks?) and requires a PDF file. So I use Illustrator to create the PDF print file with registration marks for cutting, etc. I've identified that it adds about 20% too much black to darker colors. Is there any way to decrease the black (or any of the CMYK values) of everything? Sometimes the label is just a jpg, sometimes very dense Illustrator files, or combination of both. So I can't just select and modify, I need to reduce the output of everything 20% of the K. Any help is appreciated.

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