A “codicil” is an amendment to a Will, and must be drafted and executed in the same manner as a Will itself.
If you wish to change the terms of your Will, it is very important that you not do so by crossing out words or sentences, or writing in new terms on your original Will. Doing so is likely to lead to litigation over your true intent, and your entire Will may even be determined to be invalid. Using a formal legal codicil to amend the terms of your Will can help prevent such issues.
Note that, if you have moved to California from another state and had a Will prepared in your prior state, it is a good idea to have an attorney review the Will to ensure that it is consistent with California law.