As Ankit is saying, the best way would be through a custom theme, but Olaf replied once on the same topic and explained about another (not suggested) alternative,
https://stackoverflow.com/a/53360453:
"As an
extreme, non-recommended, quickfix you might want to set the value in portal-ext.properties to
theme.shortcut.icon=../../../myfavicon.ico and place a favicon with name
myfavicon.ico in tomcat's
webapps/ROOT/myfavicon.ico.
But don't come crying later, when you accidentally overwrite it in an upgrade."
