All you would need to do is notify your current attorney that you would like to terminate services, ask for a detailed final bill, and have your retainer balance returned.
If you cannot afford an attorney, you have the right to have one appointed to you in a felony, Class A misdemeanor, Class B misdemeanor, juvenile, or direct appeal case. See Tex. Code Crim. Pro.
Telephone (800) 204-2222 or (512) 427-1463 and the extension number, as noted for each department, unless another telephone number is given.