Change the fileName variable to the next file and call doFTP again."
but, i could not understand his words.
What part of that don't you understand? The variable, fileName, contains the name of the file to send. Surely you can see that you need to change it to send a different file.
The doFTP() function performs the send. Surely you can see that you need to call the function more than once to send more than one file.