mc-cms-namelessmc/modules/Core/includes/endpoints/UpdateUsernameEndpoint.php

<?php
use Symfony\Component\HttpFoundation\Response;

/**
 * @param int $id The NamelessMC user to update
 * @param string $username The new username of the user
 *
 * @return string JSON Array
 */
class UpdateUsernameEndpoint extends KeyAuthEndpoint {

    public function __construct() {
        $this->_route = 'users/{user}/update-username';
        $this->_module = 'Core';
        $this->_description = 'Update a users NamelessMC username to a new username';
        $this->_method = 'POST';
    }

    public function execute(Nameless2API $api, User $user): void {
        $api->validateParams($_POST, ['username']);

        $fields = ['username' => $_POST['username']];

        if (Settings::get('displaynames') !== '1') {
            $fields['nickname'] = $_POST['username'];
        }

        try {
            $api->getDb()->update('users', $user->data()->id, $fields);
        } catch (Exception $e) {
            $api->throwError(CoreApiErrors::ERROR_UNABLE_TO_UPDATE_USERNAME, null, Response::HTTP_INTERNAL_SERVER_ERROR);
        }

        $api->returnArray(['message' => $api->getLanguage()->get('api', 'username_updated')]);
    }
}