Extensions are no longer given to US passports. You must renew or apply to have your passport updated.

If you were given a limited validity passport and a LETTER from the US DEPARTMENT OF STATE they will advise which form to fill out to renew your passport to its full validity. A number may be stamped on the amendment page of your passport. Examples of why you were issued a limited validity passport :

Passport lost or stolen overseas and issued at a US Embassy abroad.
Proper documentation was not given at time of emergency passport issuance.
2nd valid passport issued for a validity of two years.

Call JIDS for further clarification.