Voting Question: How do I ask for a change of venue in Texas?

My ex husband’s attorney is in house counsel for the san antonio fire department and san antonio police department. She is always married to a SAPD police officer and is running for Probate Judge this term. I have no attorney and feel that their will be a serious bias and conflict, in my custody suit within the court system. My ex wants sole custody and although I know he won’t get it because our case has no child abuse. I am a school counselor, educated etc… but I feel that with her position, favoritism will be on her side. I don’t believe it would be a fair suit, also a an Amicus attorney who is also used by the court system has much to rights, along with reputation within the system. How do I petition the court for a change?