You can use the 'update URL' link in the category where your site is actually listed to do that. This form can be used to request changes of the URL, or the modification of title and/or description. If you want a category change you should suggest the new catgory along with a reason for the change in the 'Please describe the nature of your changes:' field.
In most cases update requests are only accepted if the scope of a site has changed (i.e. a photography site in Arts now offers prints for sale, and should therefore better be listed in Shopping), or if an error has been made somewhere.
<added>Effectivement, dfy a raison, si vous préférez parler français, il y a un forum francophone. Je n'avais pas vu que vous étiez de Nice. <img src="/images/icons/cool.gif" alt="" /> </added>