Each category has custom options provided by our theme.
Edit Category page and you will find the options as follows:
Image for category heading allowing to set an individual image for a category page;
Image (icon) for categories navigation on the shop page – Categories in page title Icon to display in the main menu (or any other menu through the site)
You need to insert the icon there. Icons are taken from different sources, for example, https://www.flaticon.com/