UI3 is a powerful, modern HTML5 web interface for Blue Iris. As of April 20, 2018, UI3 is Blue Iris's default web interface for non-IE browsers and is included with Blue Iris versions 4.7.3 and newer.

UI3 has a dedicated thread on the ipcamtalk forum, here:

https://ipcamtalk.com/threads/blue-iris-ui3.23528/

If you wish to manually install UI3 (maybe to get a bugfix without waiting for it to arrive in a Blue Iris update), you can get it from the releases tab: https://github.com/bp2008/ui3/releases

To install, just extract everything to Blue Iris's www directory and overwrite all files. The default path to this folder is C:\Program Files\Blue Iris 5\www.