You can find the instance running in background/apps, by going to Task Manager (Ctrl + Shift + Esc) and there, if you find the AEM instance running in Apps as Java(TM) Platform SE binary, you can click on "End Task" after right-clicking on it.
ProTip : If you need to manually kill the java process . ENSURE that AEM is not writing to disk or you risk data corruption.
So a quick way to do this is to check the segmentstore /crx-quickstart/repository/segmentstore and view the last modified timestamps on all the TAR files. Ensure that it hasn't written to disk in the last few minutes before you kill the process. Otherwise when you bring it back up you could have issues trying to mount the repository.