Source

You are only able to renew a passport that is:

• Issued within the past 15 years
• Issued when you were 16 years or older
• Undamaged and able to be submitted with renewal forms
• Issued with your current name

If these don't apply to you, then you must submit an application for a new passport.  Otherwise, follow these steps to renewing your passport:

1. Fill out your DS-82 form, which can be found under "Step 1" on the government's travel page.
2. Sign and date your DS-82 form
3. Mail in the DS-82 form to the address located on the DS-82 form with required auxiliaries:
   
   • Most recent U.S. Passport
   • 2 identical passport photos (2x2 inches)
   • Passport fee