how can i remove the default category icon used to make it blank

Sorry not clear what you mean

in the demo version all categories icons are different, but when i uploaded to my website the categories icons for all sae with an arrow key.
why the category icons are not appearing in my site similar to demo version.

hope you understood my problem.

Define your categories in Settings > Categories

how can i remove or replace the current default ICON from the categories?

i tried to edit categories Icons.  I can change color.  But I do not know what code to put each category like Automotive (Car Icon) how to put in the edit side.  Any code should I put in the category edit space provided?

Depends on theme what it expects and support.