Set another viewer for auctex in emacs

>> Open an tex file;
>> LaTeX -> Customize AUCTeX -> Extend this menu;
>> Then LaTeX -> Customize AUCTeX -> TeX Command -> TeX Output View Style;
>> Find a line somehow like ''Extension: ^pdf";
>> Change Command to viewer you want. e.g. evince %o %(outpage) to okular %o %(outpage);
>> Click “Save for future sessions”.