Look at your local rules of practice. You would generally file an amendment to the complaint. Los Angeles, for example, has a specific form for this purpose.
You use the summons you have now. At the bottom, you check the box where it says the person was served as Doe 1.
Yes, once the defendant's true name is discovered, you must amend the complaint. San Diego has a form as well. Check the forms on your court's website.
The information/answer is not, nor is it intended to be, legal advice. Consult an attorney regarding your individual situation. This attorney is only licensed to practice law in California. Your question and this answer do not create an attorney-client relationship. Do not send/post any confidential information. For more information, visit: www.thesandiegolawfirm.com