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where are the html/php files located for the server page
In various places inside the /root/Beagle_SDR_GPS/web directory. But you can't simply update the files there and have the changes propagate to the server when it's running in the usual background (daemon) mode. This is because, for speed, all the served web files are encoded within the running binary of the server itself. That way they are transferred directly from main memory to the network with no reference to the filesystem which would otherwise compromise realtime performance.
So if you change a file you have to run the entire server build process and the Makefile will take care of re-encoding the files into a new copy of the binary. Then you restart the server.
Exception: If you run the server in development mode instead of daemon mode then the actual files in the /web subdirectory are used. But development mode is not a daemon mode. You run it manually during debugging. So you can't run the server long term this way, serving from the actual files.
You will find details given by jks on how to recompile and restart after one of the web pages is changed here: