There is no climax in the story unless one interprets the final daydream as a turning point in Mitty's life. For example, having himself executed by a firing squad could suggest that he has decided to end his excessive daydreaming and attempt to resolve the problems that cause his daydreams. Among his morally acceptable options are (1) to see a psychiatrist, (2) to take action on his own (such as becoming more assertive), and (3) separating from his wife. Among his morally unacceptable options are murdering his wife or killing himself.
.......However, given the tone of the story and the meekness of Mitty, it is likely that he takes no remedial action of any kind but simply continues to daydream and tolerate his wife's nitpicking.