@SMNCP There is no special API function to edit an existing file, you could take the details of the existing file and create a new file with the same details but changing the data field only and deleting the old file.
var start = function(request, response)
{
var fileId = "107524";//get the existing file id
var file = nlapiLoadFile(fileId);
var data = file.getValue();
var name = file.getName();
var folderId = file.getFolder();
var fileType = file.getType();
nlapiDeleteFile(fileId);//delete the older file
data += ",this is the appended data";//change the data
var newFile = nlapiCreateFile(name, fileType, data);//create a new file with the same details
newFile.setFolder(folderId);
nlapiSubmitFile(newFile);//submit it
}