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.
Blue Iris 4.8.6.3 shipped with UI3-70, but you can update to UI3-77 for a few improvements. Things will begin breaking if you update a Blue Iris 4.x installation beyond UI3-77.