Prior to Stable Build 70.x, to close an open app or window, you would pull down from the top of the screen when in tablet mode and an option to minimize, maximize, and close would appear. With Stable Build 70.x or later installed, there’s a new way to gain access to the window close action.
To close a window or app…
Pull down from the top of the screen while in tablet mode, and drag the window to the middle of the screen.
Once you let go of the window it sticks to the middle of the screen and contains the ‘X’ for closing the window.