First, you need to open Bluestacks on your computer. Once it's up and running, you'll see all your installed apps on the home screen. From there, you can long-press on the app you want to delete, and a menu will pop up with the option to uninstall.
Alternatively, you can also go to the Settings app within Bluestacks, then click on Apps, and find the one you want to delete. From there, you can click the uninstall button, and it's gone!