curl --request PUT \
  --url https://api.kit.com/v4/custom_fields/{id} \
  --header 'Content-Type: application/json' \
  --header 'X-Kit-Api-Key: <api-key>' \
  --data '
{
  "label": "Last name"
}
'

require 'uri'
require 'net/http'

url = URI("https://api.kit.com/v4/custom_fields/{id}")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Put.new(url)
request["X-Kit-Api-Key"] = '<api-key>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"label\": \"Last name\"\n}"

response = http.request(request)
puts response.read_body

const options = {
  method: 'PUT',
  headers: {'X-Kit-Api-Key': '<api-key>', 'Content-Type': 'application/json'},
  body: JSON.stringify({label: 'Last name'})
};

fetch('https://api.kit.com/v4/custom_fields/{id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.kit.com/v4/custom_fields/{id}"

	payload := strings.NewReader("{\n  \"label\": \"Last name\"\n}")

	req, _ := http.NewRequest("PUT", url, payload)

	req.Header.Add("X-Kit-Api-Key", "<api-key>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.kit.com/v4/custom_fields/{id}",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "PUT",
  CURLOPT_POSTFIELDS => json_encode([
    'label' => 'Last name'
  ]),
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/json",
    "X-Kit-Api-Key: <api-key>"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

import requests

url = "https://api.kit.com/v4/custom_fields/{id}"

payload = { "label": "Last name" }
headers = {
    "X-Kit-Api-Key": "<api-key>",
    "Content-Type": "application/json"
}

response = requests.put(url, json=payload, headers=headers)

print(response.text)

{
  "custom_field": {
    "id": 27,
    "name": "ck_field_27_last_name",
    "key": "lastname",
    "label": "Last name"
  }
}

{
  "errors": [
    "The access token is invalid"
  ]
}

{
  "errors": [
    "Not Found"
  ]
}

{
  "errors": [
    "Label can't be blank"
  ]
}