Modifying TMY3 .CSV weather file

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Hi, we are trying to modify a TMY3 weather file to reflect actual weather data collected on site for a period of one year. Can you advise as to what is the proper method to do this? What software should I use to preserve the file format?
At the moment we are opening the file with excel modifying T(DB), T(WB) and RH% cells and saving it as a new *.CSV file. Somehow by doing this we are corrupting the format.