Next/Previous post navigation problem

I would like to have the next/previous post links operation *only* within the same categories. I did find a page with reference to a toggle (“in_same_cat”) that can be set so that it will operate as I’m hoping … but I’m too much of a newbie to understand where to look to find that parameter to change the setting. I get that it would mean editing a PHP file, but I’ve looked at most on them in my installation, and can’t find it.

Anyone know where, specifically, to look? Or is there a plugin that can make this option a little less geeky to resolve?

Thanks!!!

Mark