I'm having a hard time imagining this.. Any error codes? Open the store > type in the app name or game you downloaded previously > select appropriate option > this will take you to the app/game's individual page > if the app is installed, you should see a button marked "open" where the "install" button previously stood
Edit: a different approach.. start menu > all apps list > tap on one of the Capitalized letters > Select "X" > should jump to all apps installed that start with "X" so in this case we're looking for the xbox one app > click to open
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