To be Frank, I'd have to change my name.

To be Frank, I'd have to change my name.

This is a pun. Frank can be both a first name, and a adjective meaning open and honest. The phrase "to be frank" is usually used when delivering bad news to someone, so the person hears the start of the joke, and then prepares for the bad news, and then gets a punchline instead.

To be funny, it is best to say the start of the joke slowly, and with a pained expression, indicating that you regret saying what you are about to say.