Change text string

We don’t have a snippet to change that text specifically, but you should be able to change it with a localisation plugin, such as Loco Translate. Alternatively, if you prefer to do it via code, you can write a filter for woocommerce_checkout_fields (see https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/) and change $fields['billing']['customer_location_self_certified']['description'].