You would have to contact the court where the conviction occurred.

Under Penal Code §1203.4, a person who is granted probation can apply to have the case dismissed after probation is complete. This is sometimes called an expungement, but it doesn't really expunge anything or seal your record. The conviction is still a part of the court's public records, which will also show the case was dismissed. You still must disclose the conviction when applying to be a police officer or for certain other jobs, or if asked on an application for a state license or to run for public office. It will not remove any restrictions on your right to own a firearm or relieve you of the requirement to register as a sex offender. The conviction can still be considered a prior offense; for instance, a prior theft conviction could make any future petty theft a felony, or a prior DUI conviction could be used to increase the punishment in subsequent DUI convictions.

BEWARE of law firms that promise to "clear your record" and charge thousands of dollars. The process of filing a Penal Code §1203.4 petition is pretty simple, and you can do it yourself with forms available from the court clerk's office, especially if you completed probation without any problems. If you had a probation violation along the way, you may want an attorney's help, but the guys who charge thousands to file simple paperwork will probably overcharge you for that, too.

Many convictions can be 'expunged' by proper application and Motion to the court, but only if there was no prison time served or even sentenced, if it was not for certain sex and DV crimes, if you completed all terms of probation, and if you have no new charges pending. If successful, the conviction would be retroactively withdrawn and the charges dismissed. That does not 'remove' the conviction, but merely changes the record to show 'conviction reversed and dismissed by expungement'. If expunged, you would be able to say 'no' to conviction on most employment applications. If you’re serious about doing so, and you think you qualify, feel free to contact me for the legal help you'll need.