Board Thread:General Discussion/@comment-26475270-20150831034015/@comment-26927936-20150831234146

Delete the incorrect category from each page. You do that by looking at the top of screen where you're editing and clicking on those three lines near the cancel button (if you're using visual editor, which you most likely are) then you should see a 'categories' option. Click that and from there you can delete the incorrect category and add the correct one. Just do that to all the pages and you'll be fine.