We don't currently support a windowed mode, but I'll look into releasing a small update that includes the option. Hopefully I can have it available later today!
The game's been updated! Just choose settings on the start menu (or, after you've started the game, hit pause on the textbox or press the escape key and then choose "settings") and un-check the "full screen" toggle.
no worries!! we did find few small bugs with the current version after the update, so we'll be putting out another one later today, but the windowed option should fully work right now!