You’ll need to override 2 classes to customize the push notification, this is described in the doc here: https://github.com/Backendless/Android-SDK/blob/docs/docs/push.md
Note that in this case you’ll need to build the notification UI by yourself, the tutorial could be found here: https://developer.android.com/training/notify-user/build-notification.html
You can also view the default implementation in the default receiver class (method handleMessage): https://github.com/Backendless/Android-SDK/blob/master/src/com/backendless/push/BackendlessPushService.java