According to LegalZoom the first step is to file a Removal Petition:
Removal Petition
"The grantor, any beneficiary or any co-trustee may petition a court to remove a successor trustee if proper grounds can be established. In some states, such as California, a court may initiate removal proceedings on its own motion. If the trustee refuses to resign, the court convenes a hearing and allows both sides to give testimony and call witnesses before ruling on the removal petition."
