It is easy to change the icon for a single instance of a launcher or folder.
Right click on the application, folder or document icon. Then click on the icon shown, move to a different directory and add that icon.
Icons tend to be in /usr/share/icons
It is much harder to change the icon for a file type.
I want to change pdf documents to show my favorite pdfxchange viewer.
It seems the only way reasonable is to replace the existing icons.
Unfortunately, for the default linux mint 18 default pdf viewer atria, the default icon is an svg vector graphics file and I don’t have one for the pdfxchange viewer.
So no go here!