How do I fix an a/c that will only blow out the defroster even after changing the selector switch

it's not the selector switch!you have a vacuum leak.raise the hood,look next to the fire wall passenger side at the smaller vacuum hoses.they may branch off and go to different places.do this with the engine running and the a/c on.listen for hissing noises.check all vacuum hoses,but most of the time it will be the smaller ones.when you have a vacuum leak,the blower motor will default to the defrost mode.check hoses under dash if other hoses are ok under the hood.