I played the newest version of minecraft and then I played on the older version on the same world but when I closed the game and opened mincraft launcher agian the newest version was gone

I PLAYED THE NEWEST VERSION OF MINECRAFT AND THEN I PLAYED ON THE OLDER VERSION ON THE SAME WORLD BUT WHEN I CLOSED THE GAME AND OPENED MINCRAFT LAUNCHER AGIAN THE NEWEST VERSION WAS GONE I CANT UPDATE IT ITS NOT AT INSTALLATIONS OR THE BUTTON ON THE LEFT OF THE PLAY BUTTON.HOW CAN I FIX IT