There are specific instructions on how to upgrade the firmare they can be found on the pioneer website this is a direct link to the PDF file with the instructions.
There is a button combination that must be triggered to prepare the mixer for the update. I only know this because I had to do it for two mixers since I took my first one back. I am having a feedback noise issue with the DJM-S9 and serato seems to be a USB issue as it happens on internal mode too even if turntables aren't connected. My own post explains it more found here: