Get JPEXS FFDEC, open the game's main swf (the one that you use to run the game) with it.
You'll see a list of sort on the left side of the software, look for the MainTimeline from the 'scripts', I've highlighted it as example, save format is the same with all JSK flash games, so it won't be hard to find.
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Then look for this part from the chosen .AS script; "save_data_".
It will bring you up to the right location that needs to be edited.
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If this line of code is present there, just erase it from it and the game's gallery should revert back to pre completion state, unless there is a save appdata present in the pc that all the relevant data is loaded from, if there are other stuff present such as stats, skills and the sort on the same string of code, changing those values to 0 should give you clean slate on stats and skills, but if those are already at 0, don't touch em.
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The values above are stock values of the said skills and stats.
But before you make any edits, make a backup of the main swf, because if something goes wrong, nothing of value is lost.
To save edits in FFDEC, one must first save the edits from bottom middle save, and then finally from top left save.