Retain mailing content in the Cleaver Reach using the Update API in Magento 2

I am using the Cleaver Reach API in Magento 2. I used the create mailing API to create the mailing content using PHP curl. I need to update the mailing content using API. I assigned the template in the clevereach dashboard to the email content section. Now I want to update the mailing content using API and also want to retain the template data in the mailing content. I am using the following code to update the mailing content:

/* update the mailing content by id*/
        $cleverreachmailingurl='https://rest.cleverreach.com/v3/mailings.json/<id>';
        $this->_curl->setOption(CURLOPT_HEADER, 0);
        $this->_curl->setOption(CURLOPT_TIMEOUT, 60);
        $this->_curl->setOption(CURLOPT_RETURNTRANSFER, true);
        $this->_curl->setOption(CURLOPT_CUSTOMREQUEST, 'PUT');
      $headers = ["Content-Type" => "application/json","Authorization"=> 
      "Bearer Bearer <token>"];
      $params='{
    "content": {
        "type": "html/text",
        "html": "<h4>Test 15</h4>'.$rr.'",
        "text": "Test content for the code"
    }
}';
        $this->_curl->setHeaders($headers);
        $this->_curl->post($cleverreachmailingurl, $params);
        $response = $this->_curl->getBody();
        $datafindAddressApidecode = json_decode($response, true);
        /* update the mailing content by id end*/

This is the body response i need to update the body_html section but when i getting the decode value for the body_html then showing the /n/n/n/t values.

I am using this API to update the cleavreach email content. https://rest.cleverreach.com/v3/mailings/template.json/> But when I update and add the new data the old template data is not retained.