There are currently two ways to do this depending on the client you are using.
In order to switch the game to windowed mode, press "Ctrl+Shift+F" and it should toggle it on-and-off.
If you are using the Steam version of the game, you'll have to do the following:
1) Go to Library and locate Duelyst.
2) Right-click Duelyst and go to Properties.
3) Click on "Set Launch Options".
4) Type in "-sw" without the quotations.
This will allow the game to launch in windowed mode.