To update custom property issue a PUT request to the /ccadmin/v1/itemTypes/promotion/{id} endpoint on the administration server with the ID of the existing custom property. The following example changes the label displayed for the campaign property:

{
"id": "promotion",   //this is the ID of the item-type item
"specifications": [
    {
      "id": "campaign",  //this is the ID of the property
      "label": "Discount Source"
    }
]
}