David W. answered 05/21/15
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First, I'll assume that you are not trying to do this manually (but editing using a computer), but want a computer program that can do it -- a program that can open an edf file and then save it as a csv file.
Now, determine whether you have the program that originally created the edf file. If it can "save as ..." in csv format, life is easy. Or, if the program that is to use the csv file can input an edf file, life is also easy.
However, if not (and for decades there has been a thriving market for such file conversion programs), then you must use a program that will simply input an edf file and output a csv file.
Choose that program carefully (because programs change, file formats change, amateurs often write such programs,...).
Some programs operate from their web page (and do not require downloading), but if you Google (or Bing) to search for a program, be careful of downloading it (because they often include nasty stuff, called malware, with any program that people desire).
I checked and found lots and lots of such programs (but did not download any).