on a computer that is able to connect to the internet using the livebox (horrible things BTW) click on the link below (or type it into a browser address bar)
http://192.168.1.1
if it is still at default it will be :- USERNAME = admin PASSWORD = admin
On left hand side click on configuration , In the sub selection click on wireless.
you will need to have a look round in the wireless settings, you should either be able to find a way of viewing what the current key is, or there should be an area to enter a new key that (if you enter a new key you will need to reconnect all wireless devices that use the router)