How to replace icon

These icon file are hard coded into the core at lots of places… in this file …/apps/frappe/frappe/public/icons/timeless/symbol-defs.svg

You could probably add additional icons to the above file like how its done here

When that is done just do

bench build

and

bench clear-cache

You should able to select you icons.

However do note unless its in a separate app it will break when you update your ERPNext / Frappe