The following sentence is in a business letter.

To be fit, you should get enough sleep, exercise regularly, and eat a healthy diet.

Since it is a business letter, would "you" be an unclear pronoun?

Yes. How would you make it more formal?

I would replace you with one.
Would that be correct?

Yes. You should say, ". . . one would be fit. . . ."

Actually, using "one" in a business letter may sound overly formal and outdated. While it is important to maintain a professional tone in a business letter, it is also essential to strike a balance and use language that is clear, concise, and approachable. In this case, rather than using "one," it might be better to rephrase the sentence to address the reader indirectly. For example, you could say, "To achieve fitness, one should ensure they get enough sleep, engage in regular exercise, and maintain a healthy diet." This way, you maintain a professional tone without sounding overly formal or distant.