If you want to change your stream from Non-medical to medical in 12th class, then you have to apply for it because the board registration certificate is done. So you can't change your subjects at this moment, If you wish to be eligible for the Medical courses then you can pursue the Biology as a on demand examination from NIOS. Apart from this, you can also contact your college, if there are any ways available for change.

For more information regarding NIOS consider the link below:

https://school.careers360.com/exams/nios-class-12

For more information regarding NEET consider the link below:

https://medicine.careers360.com/exams/neet

I hope this helps. Have a great afternoon!