curl --request PUT \
  --url https://api.satsuma.ai/cart/{cartId}/item/{itemId} \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '{
  "quantity": 2
}'

{
  "data": true,
  "messages": [
    {
      "context": "<string>",
      "text": "<string>",
      "data": {}
    }
  ]
}