This is a step by step tutorial to add cart icon to your main navigation menu (depend on your theme menu position).
Install and activate WooCommerce Menu Cart (Plugins > Add new > search “WooCommerce Menu Cart” > Install > Activate).
Create a new menu, go to Appearance > Menus > click “Create a new menu” > name your new menu “Cart” > Save
Go to WooCommerce menu on your dashboard > Menu Cart Setup.
On this tab : Select the menu(s) in which you want to display the Menu Cart, select the menu you just added in the previous step.
And then checked / activate :
See this image bellow for reference :