To only change the name listed on a passport, no letter is required. Go to the US Passport Help Guide to learn about correct forms to complete and mail in.
To change the gender marker listed on a passport, a doctor's letter stating the person has undergone “clinically appropriate treatment” for gender transition must be provided to the Passport Office. Please see the attached template letter. The letter must be the original, on the doctor’s office letterhead, and signed (or co-signed) by a MD-level provider. This letter, with a completed form (DS-11) must be presented to the Passport Office in person, even if they would otherwise be eligible to renew by mail. Find a Passport Office near you.
For detailed information on how the process works, review the Transgender Law Center's Guide, ID Please.