Unfortunately, even administrators can't rename a category. But I came up with a 'trick': you could delete the category from the linked article/s and add the later instead, and then once you're done, I'll to delete the previous two.
The deleted categories too, will keep on showing up for one or few days before they completely disappeared, based on my experience some-time-I-don't-even-wanted-to-remember-when ago.
Check out your Admin Dashboard, which can by found by clicking "Admin" on the bottom toolbar -- it has links to all your new tools. Please let me know if you have any questions, and good luck with your new (old) wikia!