Change icons of application in Linux Mint

One way of finding the location of the icon for an application is to add it to the panel (right click > add to panel) and then right click on the newly added icon to edit it. By clicking on the icon in "Launcher Properties" you'll get its location. For instance mintInstall is found in /usr/lib/linuxmint/mintInstall/icon.svg

Having this you can then replace the icon with you own file and you can remove the application from the panel again.

On newer versions of Linux Mint (tested on 19.1) the accepted solution doesn't work. You have to go to /usr/share/applications with elevated privileges (command: sudo nemo) and then modify the icon from there (Right click on icon you want to change -> properties -> click on the icon in upper left of dialog).