Change macOS Mojave system icons
The manual way is to go to the system folder using finder go to folder command 
shortcut Shift Command G ⇧⌘G

/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/

Rename the original icon and then name the new icon file with the original name. 
Will need to be an admin user and enter your password when doing this

Using a dedicated program may be easier see Liteicon for a free download.

The file will need to be a Mac icon file .icns 

I create my icons with Affinity Designer export to a 1024 X 1024 PNG then use iConvert Icons to make the icon files.
Any photo or vector graphics tool will work and there are other converters.