To delete an app, you'll need to head to the Home screen of your Bluestacks emulator. From there, click on the All Apps button, which will show you a list of all the apps you've installed. Now, find the app you want to delete and long press on it until you see a few options pop up.
One of those options will be Uninstall, which is exactly what you're looking for. Click on it, and you'll be asked to confirm that you really want to delete the app. Just click OK, and the app will be gone in a flash! It's that simple.