You can update a buyer by calling the PUT /buyers/{buyers_id} API. The call accepts an optional update to the external_identifier (for example the user’s email, username, or a database ID) and an optional update to the display_name which will be shown in the admin panel. 
curl -i -X PUT "https://api.example.gr4vy.app/buyers/fe26475d-ec3e-4884-9553-f7356683f7f9" \
    -H "Authorization: Bearer [JWT_TOKEN]" \
    -H "Content-Type: application/json" \
    -d '{
        "display_name": "John D."
      }'
The API returns an updated buyer resource.
PUT /buyers/fe26475d-ec3e-4884-9553-f7356683f7f9
{
  "type": "buyer",
  "id": "fe26475d-ec3e-4884-9553-f7356683f7f9",
  "external_identifier": "412231123",
  "display_name": "John D.",
  "created_at": "2013-07-16T19:23:00.000Z",
  "updated_at": "2013-07-16T19:23:00.000Z"
}