What are the proper commands? I am using Zorin 18 Pro.
Right-click on the icon you want to change, then select "open with other application" and select texteditor there.
Then the texteditor will open the .desktop file of the launcher.
Go to the line which begins with
Icon=
and set the path to the image you like to use (you can copy the image and paste it there). Then save the file.
Do you want to replace single desktop icons or all folders icons?
If you want to replace all folder icons you can take a look at this thread: