This action can only be done on the desktop application.
From the desktop app’s dashboard, click the menu icon.
Then in the menu, click “edit type” under the type you wish to edit.
Edit the type’s icon by clicking on the icon (1), then typing in a keyword for the new icon you’d like to use, such as “smile” (2), then click on an icon that appears to use it (3). When you’re happy with your changes, click “save & close” at the bottom to save your changes.