If your ESTA has expired, you must reapply by submitting a new application Here. Existing authorizations cannot be extended.
You do not need to wait until your ESTA expires to reapply. You may do so at any time. 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 to begin your new application. Your prior ESTA will be canceled and replaced with your new application.
ESTA approvals are granted a two-year period or until the applicant s passport expires, whichever date comes first. 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.