How do you reverse a legal separation?

File a Request for Dismissal.  This is a form you can obtain from the court.  On the form, check the boxes indicating that you are dismissing all parties and all causes of action.  There may be a box which says dismiss entire case or dismiss entire action or words to that effect.  Check that box.  You will also see boxes that say with prejudice, without prejudice.  You will need to check either with prejudice or without prejudice.  With prejudice means the case cannot be reinstated.  Without prejudice leaves open the possibility that the case can be reinstated in the future.  If you never want the case reinstated, check dismissal with prejudice.  If you want to keep the option open of possibly reinstating the case sometime in the future, check dismissal without prejudice.

Both parties should sign the Request for Dismissal where indicated and then file it with the court.  When you file it with the court, some court clerks in CA don't immediately stamp it as filed while you are waiting.  So, include a self addressed stamped envelope in order to receive a conformed (court stamped) copy for your records.