You are required to follow the same directions as getting your learner's permit for the first time. Meaning, present to the DMV all necessary documentation, pay the application fee, and pass a written test. It is recommended that you make an appointment first. You can reach the DMV at 800-777-0133 or visit their website at http://www.dmv.ca.gov.
If you are under the age of 17½ read the following carefully, IT'S IMPORTANT! When renewing your learner's permit make sure to keep your expired learner's permit, DO NOT THROW IT AWAY. This will prove to the DMV that you've had your learner's permit for at least 6 months. The DMV only maintains a record of the last learner's permit issued and all previous information is erased. Therefore, if you don't keep your expired learner's permit it will appear to the DMV as if you received it on the date of your new learner's permit and you will be required to wait 6 months from that date or reach the age 18+ in order to be eligible for your drivers license.