In this tutorial, we will, as the title suggests, see how to append to a file in and SD Card connected to Arduino. Actually, it is quite simple. If you have gone through any previous articles on SD Card, then you only need to know that
myFile = SD.open("example.txt", FILE_WRITE);
opens example.txt in the append mode only. Thereafter,
appends to the existing file, and doesn’t overwrite the existing content.
If you haven’t gone through any other articles on SD Card, I’d suggest reading the "Store a new file in SD Card connected to Arduino" article. That is a detailed article containing the circuit diagram and also a Code Walkthrough. That article will make this appending part very clear.
If you wish to overwrite the content, that’s not very straightforward actually. But solutions are available. You can follow this discussion thread for details on how to do that − https://forum.arduino.cc/t/overwriting-values-on-sd-card/543451