since you have already registered for their postutme screening exam there is nothing you can do again.
You can't change the course now because you have already registered for their postutme screening exam.
Buk jamb subject combination for public administration is;
English Language(compulsory for all course),Government or History plus two other Social Science subjects.
They might accept the hausa as social science subject.

Your UTME subject combination for public administration in BUK is correct, because the standard laid down for the UTME subject combination for studying public administration in any university including BUK is English, Government, Economics, and any other subject. So there is high chance of you gaining admission into BUK if you aim at and score high in your post utme examination. You can't do any change of course since you have applied for post utme, but don't worry the UTME subject combination you chose is correct.