In Texas, if you cannot afford to hire an attorney, and if you are charged with a crime that is punishable by incarceration, an attorney will be appointed to defend you. Once a public defender has been appointed to defend you, you may ask the court to appoint a substitute attorney only for good cause. Good cause requires more than mere dissatisfaction with your appointed attorney and may include:

A conflict of interest between you and the attorney
Your attorney becomes ill and cannot continue to represent you
There is reason to believe that your attorney is not providing effective assistance.