The mistakes in form filling happen frequently. 

It is advised to email the authorities immediately stating your application ID and the details of the correction to be done.

They would surely help you out in correcting the form.

And do it before the exam and get an email reply from them.

All the best.

Hello! Please mention the exam you are applying for. Some exams do have an application correction window. Check the important dates section for that. Moreover, in case there's no such option, contact the exam authorities.