How to upgrade WordPress automatically?

By default WordPress automatically updates itself, plugins, and themes.
If your WordPress install doesn’t do that then you probably have something that disables this.

You can check your theme for something like this

add_filter('auto_update_plugin', '__return_false');
add_filter('auto_update_theme', '__return_false');

And/Or in your wp-config.php

define('AUTOMATIC_UPDATER_DISABLED', true);
define('WP_AUTO_UPDATE_CORE',        false);

If your WordPress is not on a local machine and is on a hosting env, and you can’t find anything like the code above, you can try contacting your hosting about this, they might provide additional help.