First, make sure you are referring to the correct file location.
"C: \ Users \ [UserName] \ Appdata \ LocalLow \ Glitch Picth \ Idol Manager \ data \ manual_saves \ [saveID] \ save.json"
Then open the file in a text editor and search for the string "skill".
You will find data consisting of four rows: "skill_type", "primary", "exp", "lastFrameWorking".
Of these, "exp" is the experience value of staff skills. You can change this value.
Skill MAX requires 500,000 points, but since the judgment flag of whether it is possible to level up may not work properly,
I change the exp of the two skills to 499999, then load that data in the game and try to give it some task to get experience points.