Uninstall Opera, choose not to delete your data. Download http://ftp.opera.com/ftp/pub/opera_gx/71.0.3770.205/win/Opera_GX_71.0.3770.205_Setup_x64.exe, launch the Opera GX installer, click "options", set "install path" to "C:\Program Files\Opera GX", set "install for" to "all users", uncheck "import data from default browser", check "make Opera default browser", and install. Then, you should be able to make Opera GX the default browser in Windows.