In Windows 8, it is very easy and quick to close running Modern UI apps. You only need to just click and drag the running app from the top edge of the screen to the bottom of the screen on desktop PCs or just swipe in touch screens. However, this has changed a little with the Windows 8.1 update.
In Windows 8.1 preview, to close a running metro app (Modern UI apps) you have to drag the app down the screen and hold it for a while to get it closed. You will see the app rotates and this will close the app completely.
However, note that the closed app will not completely close immediately. It still runs in the background for about 10 seconds and closes automatically – no further actions are required. It can be useful if you want to open the app quickly again.
Another way to close a running metro app or any app in Windows 8.1 is via Windows Task Manager. Just open the Task manger (Ctrl + Alt + Del) and under Process tab, kill required apps. (via)