documentation/extensions/odoo_theme/layout_templates/version_switcher.html

<div class="o_versions row">
    <div class="dropdown">
        {%- if alternate_versions|length > 0 %}
            <button class="btn btn-sm dropdown-toggle"
                    id="versions"
                    data-bs-toggle="dropdown">
                {{ version_display_name }} {# The current version #}
            </button>
        {%- else %}
            <button class="btn btn-sm dropdown-toggle"
                    id="versions"
                    disabled="">
                {{ version_display_name }} {# The current version #}
            </button>
        {%- endif %}
        <ul class="dropdown-menu dropdown-menu-end" aria-labelledby="versions">
            {%- for alternate_version, url in alternate_versions %}
                <li><a class="o_switcher_item dropdown-item" href="{{ url }}">{{ alternate_version }}</a></li>
            {%- endfor %}
        </ul>
    </div>
</div>