How did you train the classifier? 40% accuracy on a binary classification is worse than random (would be 50% if the class are balanced or higher if they are not)
Anyway, combined with changing the trashold, that that is done after you have altready trained the classifier, if you have unbalanced (but that’s usually high accuracy and low recall of the minority) consider oversampling the smaller class or undersampling the other. Alternatively you can weight the loss function (or the gradient); this can also bo done in case predicting well a class is more important than the other.
But I’d check the classifier first.