Hello Gokul

Yes you may change your mode of course but for that you must have studied biology in class xii , if not then you may give biology exam through any of open universities.  Many universities in India provide this facility that a candidate who wish to appear of singke subject exam or droppers also proceed through open universities. One of the best option is NIOS . It is National Institute of Open Schooling.  NIOS marks are accepted in all universities of India.

Eligibility criteria for appearing Neet is that

The candidate must have completed 17 years of age. The candidate must be an Indian.

The student must have passed class xii with minimum 50 % marks in physics, chemistry and biology from any reputed university.

For details please visit the site given below

https://medicine.careers360.com/articles/neet-eligibility-criteria