In default WSO2 IoT server, you can't uninstall mobile apps from the apps store. But, you can self-unsubscribe from mobile apps by changing a config. For that, you have to set "EnableSelfUnsubscription" as true in <IoT_HOME>/core/repository/conf/app-manager.xml
Then, restart the server.
Login to store and click on "My Apps" tab. Click on the button (with 3 dots) in the bottom right corner of the app and click on "Uninstall".
That's all. :)