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I've tried this and putting "Icon-36.png", "Icon-48.png", and "Icon-72.png" into the application folder AND the data folder and none of it works, help.
Check this: https://forum.processing.org/two/discussion/20443/please-help-me-urgent-p2d-or-p3d-change-icon#latest
See also recent activity on github -- not sure which icon you are trying to change, and on which OS.
I have windows 10 and am using processing version 3.3.5. I want to change the icon you see when you export the application and put it on the taskbar or on the desktop.