Hcfyk

I added some config yml files to my module's config/install directory so that it is installed directly when installing my module, it works well only if there isn't a config file with the same name installed already and only if it's the first install.
So my question is there a way to update the config files already installed with my module's config file, and delete them when uninstalling the module ?

A module install should not update config. If you want to update existing config, your module should implement hook_update_N() (see docs) instead.

However, you could add a dependencies: enforced section to your YML config files. This will remove your existing config on module uninstall, and install the newest config on module reinstall. This way you can "emulate" a simple update mechanism for your own, private custom module. See doc for enforced config dependencies

You can update custom config with a settings form you build.
In a custom module custom_mailing add a file called custom_mailing.routing.yml add the following route:

custom_mailing.route_admin_settings:

  path: '/admin/config/mailing'

  defaults:

    _title: 'Custom mailing'

    _form: 'Drupalcustom_mailingFormAdminSettingsForm'

  requirements:

    _permission: 'administer custom_mailing settings'

Now I want a boolean value to be editable. I create a file in a map config/install called custom_mailings.settings.yml with the following content:

mailsettings:

  enabled: 0

In a module called custom_mailing. Inside src/Form add file called AdminSettingsForm.php.

<?php



namespace Drupalcustom_mailingForm;



use DrupalCoreFormConfigFormBase;

use DrupalCoreFormFormStateInterface;



/**

 * Class AdminSettingsForm.

 *

 * @package Drupalcustom_mailingForm

 */

class AdminSettingsForm extends ConfigFormBase {



  /**

   * {@inheritdoc}

   */

  public function getFormId() {

    return 'custom_mailing_admin_settings_form';

  }



  /**

   * {@inheritdoc}

   */

  protected function getEditableConfigNames() {

    return [

      'custom_mailing.settings',

    ];

  }



  /**

   * {@inheritdoc}

   */

  public function buildForm(array $form, FormStateInterface $form_state) {

    $config = $this->config('custom_mailing.settings');



    $form['mailsettings'] = [

      '#tree' => TRUE,

      '#type' => 'fieldset',

      '#title' => $this->t('Email settings'),

    ];



    $form['mailsettings']['enabled'] = [

      '#type' => 'checkbox',

      '#title' => $this->t('Enable sending of e-mails.'),

      '#default_value' => $config->get('mailsettings.enabled'),

    ];



    return parent::buildForm($form, $form_state);

  }



  /**

   * {@inheritdoc}

   */

  public function submitForm(array &$form, FormStateInterface $form_state) {

    $config = $this->config('custom_mailing.settings');

    $config->set('mailsettings', $form_state->getValue('mailsettings'));

    $config->save();

    parent::submitForm($form, $form_state);

  }



}