As you have filled up the form quite a long time ago, I think it may not be changed now the exam date announced which is in coming moth. Hope that the dispatch process for the admit card also started. so, in this situation change in name may not be possible.
So, better is do not think about it so much, rather concentrate on your studies for the exam. Although you do not have specified about the mistake and what mistake it is? If it is minor then the center in charge of the examination venue can resolves this. So, carry all the original proofs such as voter id, pan card, adhaar card etc. along with their photocopies with you when you are going for your exam. Also carry a self declaration written in your own hand and do not forget to sign it. Hope it will work for you.