I assume you mean "How many psychiatrists does it take to change a light bulb? Answer: one, but the light bulb must sincerely want to change." "Light bulb" jokes poke unkind fun by mocking and stereotyping some group. The implication is that the group is stupid or lacking in common sense. I believe the original joke referred to Polish-Americans, an ethnic group common in some U.S. locales--Milwaukee, Wisconsin, for example. The original joke was "How many [derogatory term] does it take to change a light bulb? Answer: Three: one to hold the bulb and two to turn the ladder." When a joke becomes popular, sometimes it spawns a huge family of similar jokes. In the new jokes, sometimes the surprise is finding a fresh version of the same basic idea. Other times, the surprise comes in violating what the listener expects, breaking the pattern of the family of jokes. The typical "light bulb" joke involves changing a light bulb in some surprisingly complicated way that requires more than one person. The "psychiatrist" joke breaks the usual "light bulb joke" pattern. Here there's a pun on the word "change." To change a light bulb means to replace it. You take out the old bulb and put in a new one. "Change" in psychiatry means "to become different;" to recover from neurosis. Traditional Freudian therapists sometimes say that they can only help if the patient truly wants to recover. The joke is mocking that idea.

Conveniently enough, this topic has its own Wikipedia page: https://en.wikipedia.org/wiki/Lightbulb_joke

Have a read of the jokes on this site below. You might understand it better this way. http://www.lightbulbjokes.com/computer.html