There's a convenient page on the ESTA program website:
When my ESTA expires how do I renew my application?
If your ESTA has expired, you must reapply for a brand new authorization by submitting a new application. Current authorizations can not be extended. Go to ESTA.cbp.gov, follow the instructions to answer all of the required questions and submit a new application for travel authorization - just like you did the first time you applied.
You do not need to wait until your ESTA expires to re-apply. You may do so at any time before, on or after the expiration date of your existing ESTA. Should you receive the message "A valid, approved application with more than 30 days remaining has been found for this passport. Submitting this application will require payment for this application and will then cancel the existing application." you will simply continue with the application to begin your new application. Your prior ESTA will be cancelled and replaced with your new application.
ESTA approvals are typically granted for a two-year period or until the applicant's passport expires, whichever is sooner. The approval is good for multiple entries - meaning you may use it for more than one entry into the U.S. ESTA provides validity dates upon approval of the application. The associated $14.00 fee will be charged for each new application.