The .jar config file shows these following changeable options.
I wanna change the HUD but has nowhere to start it.

{
"resetOnDeath": false,
"disableAttributesGui": false,
"showLevelNameplates": true,
"skillPointsPerLevelUp": 1,
"levelFormula": "stairs(x,0.15,2,25,10,22)",
"levelUpVolume": 100,
"skillUpVolume": 100,
"textScaleX": 50,
"textScaleY": 50,
"tooltip": "PLAYEREX"
}

The PlayerEx version you are referring to does not implement a HUD.

The PlayerEx version you are referring to does not implement a HUD.
yeah, but what if I add a code into the file, will it work?

I do not understand what you are asking. You have said "I wanna change the HUD" and showed me the config file options. PlayerEx does not add a HUD. That is why there is no option to disable one in the config.

Additionally, I do not know what your reply means:

yeah, but what if I add a code into the file, will it work?

What code? What file? What will or will not work?

I cannot provide help if you do not clearly state what it is you need.