The ability to change your stats with a "bet" is indicated by how many "bet points" you have in the top left. What I'm guessing is that you can make the first bet, but then not the second because you run out of points. You can gain more points by killing enemies. Still, it lets me press Shift and then Enter and makes me jump and does a bet with 0 points. In short, Shift is to start a bet, Backspace is to cancel, W is to raise points to bet (max indicated in the top left by 'Bet Points: #'), S is to lower, and Enter is to confirm.