Hi,

For the menu the icon which has to displayed is specified like this,

<menuitem name="My App" id="my_menu" web_icon="module_name,static/description/icon.png"​/>

To change the existing menu icons, either you can do it from the Front end or from the code. If you are doing from the code, inherit the menu and update the web_icon with the new icon.

<menuitem id="module_name.menu_root" web_icon="module_name,static/description/icon.png"/>

From the front end, activate the developer mode and navigate to Settings -> Technical -> User Interface -> Menu Items, search the menu and open it. There you can see a field named Web Icon Image where you can add the new icon.

Thanks

May be this will helpful