How to change Finder icon on macOS Catalina?
Based on this answer: Cannot change login wallpaper in 10.15 Catalina?


Reboot Mac, holding ⌘R to boot into Recovery mode
Open terminal
Disable SIP with csrutil disable
Reboot Mac
Open a terminal
sudo mount -uw /
sudo cp ~/your-new-icon.png /System/Library/CoreServices/Dock.app/Contents/Resources/finder.png
Repeat for finder@2x.png
Reboot into recovery
Re-enable SIP with csrutil enable