Your thumb impression can be corrected if you have received a notification asking you to do so. So technically you should be able to make the changes. You need to make the changes as soon as possible because you cannot make the corrections after a stipulated period of time. Your corrections in the application form are n ot counted when you receive a notification asking you to make changes. So you can still correct the application form. Please follow the instructions in the mail sent to you. If it doesn't work out, please reach out to them at; fmge@natboard.edu.in  and please wait for 24 hours atleast before you hear from them. The email addressed to NBE should mention name of Candidate, application ID, correspondence address and mobile number. Emails sent at any other email IDs or not as per communication protocol shall not be processed. Or you can also call them up at: 18002674003