Hello ,
No need to worry if you have used your right thumb impression instead of right thumb impression. NTA will provide time for correction in online NEET 2020 application form from Jan 15 till Jan 31st . You can only correct your application once. You can either change in application form or even documents uploaded. You better be prepared and wait for the given dates. Don't make a mistake this time, or else no other opportunity for correction will be given.
Hope it helps you.

Hi Manu

Yes you can correct the thumb impression upload during the NEET correction period and upload you left impression.

The correction period is to start from 15th January to 31st January 2020. Most probably you will be sent a mail on how to proceed with the corrections in the application, like last year by the NTA. All the best.