Change JSON field using PowerShell Core

I have following JSON file and i need update the displayName to contain either QA or Production :

{
    "analytics": [
      {
        "kind": "Scheduled",
        "displayName": "RO-0024-DEV GPO Scheduled Task",
        "description": "test",
        "severity": "Low",
        "enabled": true,
        "query":"SecurityEvent | where EventID == \"5145\"  | where AccountType == \"User\"",
        "queryFrequency": "5H",
        "queryPeriod": "6H",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 5,
        "suppressionDuration": "6H",
        "suppressionEnabled": false,
        "tactics": [
          "Persistence",
          "LateralMovement",
          "Collection"
        ],
        "PlayBookName": "Test447"

      }
    ]  
}

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i am running a powershell script via Azure Devops build agent and depending on the Tennant, i want to change the DisplayName that contains Dev to QA or Prod.

i have tried the following for testing however it fails:

$rules = Get-Content -Raw -Path .\TestJson.json | ConvertFrom-Json
$rules.analytics.displayname = "test"

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i get the following error:

The property 'displayname' cannot be found on this object. Verify that the property exists and can be set.
At line:1 char:1
+ $rules.analytics.displayname = "test"
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyAssignmentException

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Thank you in advance.

Regards,
Kelly