Hello Aspirant,

As you told that you have mistake in your surname, you must immediately notify or contact the concerned authorities who are conducting the exam about the problem.

You can maybe mail them, contact them through phone number or some other sources as would have been given on their website.

Since, if you are unable to do so, you would maybe land in sort of  trouble since your surname on both the documents are different. But, might be contacting the authorities through their portals could definitely help you. Usually, the authorities give a helpline number or email ID which could be used by the students who are facing difficulties. So, for that you have to check on the official website.

Hope that helps. Good luck