Board Thread:Support Requests - Designing Your Wiki/@comment-4497034-20130222140811/@comment-10688548-20130228040533

You can change the displayed text in Special:ThemeDesigner, but that doesn't have perfectly strong effects, to my understanding it only changes what is seen in the absence of a formal wordmark logo, as is the case on your wiki. The old name will still be displayed elsewhere.

You should go to Special:Contact/wiki-name-change (I guess that nobody in this thread can make a proper link) and file a request for the URL of your wiki, or your Sitename, or both, to be changed. Be specific about what you want them to be.
 * The URL of your wiki is what is typed in the address bar to find it. If we're talking about w:c:isaacsguidetotheuniverse, then the URL is isaacsguidetotheuniverse.wikia.com. Note that any changes in the URL will create a redirect from the old one to the new one, so links won't break.
 * The Sitename of the wiki is the name of your site. If we're talking about w:c:isaacsguidetotheuniverse, then the Sitename is "Alienscifi's guide to the Universe".