Sorry to hear that you are having trouble with this. While the game is open and taking over the entire screen, could you please try pressing:
Windows key + Shift + Enter
This shortcut has been used to toggle supported Windows Store/UWP apps between full-screen and windowed mode. You can also try Windows key + Down Arrow once or twice to see whether Windows restores the game from its maximized/full-screen state.
If the problem still occurs, could you let us know whether Windows + Shift + Enter has any effect, and whether the game has any Display, Full Screen, Windowed, or Graphics options inside its own settings?