Navigate to Products > Categories and select a category.In the Edit Category section, find the option Icon. Click the previewed icon button to open the icon picker window, you can search for a specific icon and chose the desired one.
How to add custom category icons?
To add custom icons, navigate to Products > Categories and select a category.In the Edit Category section, find the option Icon Type and choose Custom Icon. You can upload custom icons by clicking the Upload/Add image button.