How do I contact the carrier handling my parcel?

Go to your carrier's website.
If you wish to reschedule or refuse delivery, you can in some cases do so directly from the parcel tracking page.

COLISSIMO: https://www.laposte.fr/outils/suivre-vos-envois
CHRONOPOST: https://www.chronopost.fr/fr/suivi-colis
DHL: https://www.dhl.com/fr-fr/home/suivi.html
UPS: https://www.ups.com/track?loc=en_US&requester=ST/

Otherwise, or for any other more specific request, contact your carrier's customer service department. You can find their contact details on their website.