data transfer from arduino to text file

how to transfer the flex sensor data readings from arduino serial monitor to any text file like excel document?

Format the serial output as required, perhaps comma separated Use a terminal emulator that can log its serial input to a file such as TeraTerm Load the file into Excel

or

Save the data to an SD card attached to the Arduino then move the card to the PC and read it using Excel.

Format the data properly as for a CSV file, and send it direct to Excel http://forum.arduino.cc/index.php?topic=437398.0 You don't "transfer the readings from the serial monitor", you send it to the proper destination in the first place.