To renew your passport follow the instructions at travel.state.gov.
The U.S. Department of State will no longer add visa pages into U.S. passports beginning January 1, 2016. Previously, U.S. passport holders had the option to pay for the insertion of additional 24-page visa inserts when valid passports lacked adequate space for entry or exit visa stamps. The decision to discontinue this service was made to enhance the security of the passport and to abide by international passport standards.
Requests for additional 24-page visa inserts will only be accepted until December 31, 2015. Beginning January 1, 2016, applicants in need of additional pages in their valid passports must obtain a new passport.