The icons of Linux mint are located in the directory /usr/share/icons/Mint-X
That's the path you can use, e.g., to change the icon of the desktop launchers, like this:
+ Right click on the desktop launcher icon.
+ Click on "Properties".
+ Click on the icon.
+ Paste on the Place box the text /usr/share/icons/Mint-X/ and press Enter.
+ Navigate the folders to find the icon you like.
+ Press Enter.
+ Click on Close.