This process may vary slightly, depending on your folder, but it will get you the desired results:
1) Navigate to the folder whose icon you wish to change and right-click it.
2) Select “Properties” from the dropdown menu.
3) Press the “Customize” section.
4) In the “Folder Icons” part of the section, press “Change Icon.”
5) There will be plenty of icons to choose from. Find the one that suits you, or press “Browse” to pick a custom icon.
6) Return to the previous page and hit “OK” to save your changes. The change only applies to the folder you selected. To use a different icon for another folder, just follow the same process.
Sources:
https://www.alphr.com/how-to-change-default-icons-windows-10/