WordPress PHP Change - Prev/Next Stories from Same Category

Hey guys, We've got a fairly simple task. One of the sites we're developing has previous and next links at the bottom of blog posts. Right now that links to the next and previous story regardless of categories. We want this changed so that it shows the previous/next story posted in the same category as the article that is being viewed belongs to. Example, if a user is reading a story from CAT123, the previous/next story links should only link stories from CAT123, not the whole blog. Need work done ASAP. Be realistic with costs please. Any experienced freelancer who cannot do this in less than an hour, please do not bid. It's a matter of changing the PHP query. I just do not know the right code to modify that query.