This option is available in the "Application" tab of your app editor.
For Push icon, we recommend to upload a white icon with transparency (.png).
However our system will colorize your icon in white if needed.
Note: If your icon is a square without transparency, the Push icon will be a white square.
The Push icon displays only the shape of your icon.
Example with this icon (which is a .png file with transparency) and a cover image:
Important: if you change the Push icon, your app must be republished.