By default, Odoo's menu system in the eCommerce site allows you to link menu items to internal pages within Odoo. However, if you want to link the "about" menu item to an external URL, you can follow these steps:

1. Go to the Odoo backend and navigate to the Website module.
2. Select the "Website" menu item and then choose "Menus" from the submenu.
3. Look for the menu item that corresponds to the "about" link on the eCommerce site.
4. Edit the menu item and locate the "URL" field.
5. Instead of entering a page name or path within Odoo, enter the external URL you want to link to. For example: https://www.example.com/about.

Save the changes, and the "about" menu item should now be linked to the external URL you specified. When users click on the "about" link on the eCommerce site, it will redirect them to the external page.

Please note that the exact steps and options may vary slightly depending on the version of Odoo you are using.