https://forums.opera.com/topic/2356/can-t-make-opera-20-defalut-browser-in-win-8-1-x64/6?lang=en-US
Can't make Opera 20 defalut browser in Win 8.1 x64 | Opera forums
Feb 1, 2015 - If you Opera installer, you will be able to make it default. I did that