theory test

Walaikum Assalam sister
I dont know any specific dua for exams but you can make this beautiful dua.

اللَّهُمَّ إِنِّي أَسْأَلُكَ عِلْمًا نَافِعًا وَرِزْقًا طَيِّبًا وَعَمَلاً مُتَقَبَّلاً
O Allah! I ask You for knowledge that is of benefit, a good provision and deeds that will be accepted. [Ibn Majah and others]

Also this is the dua for hardship

اللهُمَّ لاَ سَهْلَ إِلَا مَا جَعَلْتَهَ سَهْلاً، وَأَنْتَ تَجْعَلُ الْحَزْنَ إِذَا شِئْتَ سَهْلاً
“O Allah, there is no ease except in that which You have made easy and You make the difficulty if You wish, easy.”

Other tips I can give is

1) Relax yourself. Get enough sleep to maximize your performance in exam.

2) Eat healthy and avoid eating foods which makes you sleepy before exam.

3) If you have prepared in advance for exam then avoid doing last moment revision it will help you to control anxiety before exam. Although it's totally your choice if you want to do the last moment revision.

4) Enter the exam hall with confidence that you have worked so hard inshaAllah you will pass the exam.

5) Before writing anything have a quick glance over all the questions and select the questions wisely if you have given choices.
May Allah grant you success in this world and akhira.