The web pages (in html) are in C:\Freescale\KSDK_1.1.0\demos\lwip_httpserver_demo\httpserver_bm\src\fs.
They need to be converted into a file for inclusion into the project.
This is accomplished using the makefsdata tool in C:\Freescale\KSDK_1.1.0\demos\lwip_httpserver_demo\httpserver_bm\src\makefsdata
It creates the file fsdata.c which is referenced in C:\Freescale\KSDK_1.1.0\demos\lwip_httpserver_demo\httpserver_bm\src\fs.c
So to make changes to the webpages, edit the html files and run the makefsdata script and rebuild your project.