How do I change a legal separation to a divorce?

It's not clear from your question if you already have a JUDGMENT OF LEGAL SEPARATION, or if you're simply separated. If you've got a JUDGMENT of legal separation, (including division of any property and debts, child custody and support orders if you have children etc.) then that case (call it "case file #1) is essentially OVER, and can't be "converted" to a divorce. You can, however, start a new case file for a dissolution of marriage,pay the new filing fees, and the ONLY thing that will need to be done in that NEW case is dissolving the marriage; all the other stuff was done in case #1.
You'll have to re-do SOME, but not all the paperwork, since your Petition for DISSOLUTION should indicate that there are no financial issues to address because they've ALREADY been addressed in case #1.

I agree with Mr. Gould-Saltman. I would like to add what you can do if you DID NOT obtain a JUDGMENT OF LEGAL SEPARATION, if you merely filed for either a legal separtion or for a divorce but did not obtain a Judgment. Although you could try to amend the Petition, you may not be able to, because six years have passed. You have exceeded the amount of time (5 years) the court in California can leave a file open without action. You will probably have to file a new action for Divorce.

Disclaimer: I am licensed to practice law in the state of California. Therefore if your case in not in California, the information contained herein may not apply. This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.