How do I change the value of a select box field of a certain entry through the front-end?
Twig
You can check for certain URL parameters via
{% set param = craft.app.getRequest().getParam('id') %}
{% if param is not null %}
    {% set entry = craft.app.getElements().getElementById(param) %}
    {# insert your new field value! not the label, the value/handle
    {% do entry.setFieldValue('fieldHandle', 'Reserved') %}
    {% do craft.app.getElements().saveElement(entry) %}
{% endif %}

However I would not recommend doing this in Twig
PHP
You can see how to use Template Variables and create your boilerplate at pluginfactory.io
{% do craft.myAwesomePlugin.saveEntry(entry, {fieldHandle: 'yourValue'}) %}

And your function could be
public function saveEntry($entry, $attributes = []){
    $entry-&gt;getContent()-&gt;setAttributes($attributes);
    craft()-&gt;entries-&gt;saveEntry($entry);
}

That's just a &quot;quick&quot; version, I suggest you to take a look at EntriesController to see how Craft handles it in detail