@PUT("{username}")
Call<Void> updateUser(@Path("username") String username,@Body User user);

private void updateUser() {
        User user = new User("iryna", "Alala", "alala@gmail.com", "some properties" );
        Call<Void> deleteRequest = App.service.updateUser("bala", user);
        deleteRequest.enqueue(new Callback<Void>() {
            @Override
            public void onResponse(Call<Void> call, Response<Void> response) {
                // use response.code, response.headers, etc.
            }

            @Override
            public void onFailure(Call<Void> call, Throwable t) {
                // handle failure
            }
        });
    }