Navigation within category

I am using a plugin name “WP Post Navigation” Its a wonderful plugin for posts navigation which have also an admin panel for custom settings but problem is that I want to use navigation next and previous link between single category not for all posts of the blog but there is no any option for it. Anybody can help me for changing codes of the plugin the codes of the plugin are below

Main file code

[ Code moderated, just post a link to the plugin. Or for that many lines of code please use pastebin.com instead. ]

Help me please

[No bumping. If it’s that urgent, consider hiring someone.]