By default the settings.js can be found inside folder *\Clients\www\software\html5\ where * represents the path to RDPlusinstallation or its branding name path, as example
C:\Program Files (x86)\RDPlus\Clients\www\software\html5\settings.js
This settings.js is only effective when running HTML5 client with default built-in webserver.
If you use third part webserver like IIS or Apache then you should find the right settings.js file in root subpath of belonging webserver, as example for IIS by default that would be following path
or any else according to your webserver type and configured path!