I have a misdiagnosis on my medical records and my pcp is gone from the practice. How do I get it removed?

Discuss with new pcp. I would talk it over with your new pcp. Perhaps he/she can clarify the diagnosis error (or verify its validity) and open communication with your insurance company to correct it in their records if necessary.
See another doctor. You likely would need to see another doctor in the practice to get your medical records updated. While the contested report may not able to be actually removed from your chart, there could be an update made in your records if the new physician agrees that a correction is indicated. There are federal laws in place for this situation, i believe.