Legal Question in Wills and Trusts in California

You would have to file a petition with the probate court, and provide a declaration as to the reasons why you want the administrator removed.

To add to the previous answer, to remove an administrator requires a showing of good cause. The burden of proof would be on you to demonstrate that the administrator is somehow unfit, that they are not performing their duties and/or that they have engaged in affirmative misconduct. It is a fairly high standard and court's do not want to see such petitions presented to air trivial grievances. I strongly urge you to consult with an attorney to determine if the administrator in your case falls within any category giving justification to seek removal.