The short answer is You cannot simply click “Install” from a website. Your Windows Phone will reject direct .xap or .appx file installations unless you “jailbreak” (unlock) the device.
Opera Mini for Windows Phone 8 is no longer downloadable or supported via official means. opera mini for windows phone 8 download
However, this article will walk you through everything: the history of Opera on WP8, why it was special, whether an unofficial is possible today, the associated risks, and the best modern alternatives to keep your device useful. The short answer is You cannot simply click