Not sure myself. I've never found a way that works. You could try a force quit of App Store:
Press COMMAND-COMTROL-ESC keys together to open the task manager. Select App Store from the list then click on the Force Quit button.
You can also open Activity Monitor from the Utilities folder, click on the Name header to display alphabetically, locate the process then click on the Quit icon in the toolbar then click on Force Quit button. If you can identify it, look for a download process that may still be listed. If you find one then quit it, too. I don't really know if there will be one nor how to identify it. Yes, I know I'm not much help at this point, and I apologize for my ignorance.