You probably can't as the game does not support ALT-TAB. From the readme:
The Windows feature for switching between active tasks (combination of Alt + Tab keys) is not fully supported by the game. It is therefore recommended that you avoid using this combination when playing “Jack Keane”. Should you inadvertently press the combination of keys, please restart the game.
Yeah, it's Jack. Nobody alt-tabs him!
But seriously, sometimes it crashes the game, usually works just fine. Save, alt-tab, if it crashes - just launch again and load.