medium-phx-example-update.ex

defmodule Example.UserController do
  use Example.Web, :controller

  # ...

  def update(conn, %{"id" => id, "user" => user_params}) do
    user = Repo.get!(User, id)
    changeset = User.changeset(user, user_params)

    case Repo.update(changeset) do
      {:ok, user} ->
        render(conn, Example.UserView, "show.json", user: user)
      {:error, changeset} ->
        conn
        |> put_status(:unprocessable_entity)
        |> render(Example.ChangesetView, "error.json", changeset: changeset)
    end
  end
end