How to update a text file dynamically
It's difficult to answer without details of how your server works.

If your arduino is simply connected to the server, the server could open the arduino serial port (e.g.: COM5: or /dev/ttyUSB0) and simply read data.  The arduino would only need to serialWrite() data for the server to parse and store.

If your arduino is not connected but has ethernet support you could do a HTTP request to your server, and pass the data as a parameters to the request, e.g.:

http://your.server.ip/send_data?param1=10.3&amp;param2=5.6&amp;param3=foo


When your server processes this HTTP request, it appends param1, param2, param3 to that text file.

The arduino could also beam data back to your server via IR, or bluetooth, etc.

There's lots of ways to do this.