The older help file says that if you want to change the hp custom parameters type, you need to use a script call like that:

$gameParty.members()[0]._rep += value 
$gameParty.members()[0].refreshCustomParameter(paramId/shortName)

Now you can just use that to change the current parameter value, like changing the hp and not the mhp, you can use the following:

$gameParty.members()[0].changeCustomHpParam(paramId/shortName, value)

To set a value, you can use the below:

$gameParty.members()[0].setCustomHpParam(paramId/shortName, value)