How to do 12th in Biology via correspondence or online?

You can do 12th class or equivalent course via distance mode through the National Institute of open schooling. They call it Senior Secondary Course and it is equivalent to Class XII for all practical purposes.
This course is designed for students who passed 10th class and aiming to study 12th class or equivalent. The details of the course are given on their website.
A student has to select a minimum of five subjects with at least one language or at most two languages, which is compulsory for Certification. They have to successfully complete all the above subjects. Biology is also there in the subjects in group E and you can select the same as one of the subjects.
https://www.nios.ac.in/ is the website of this institute. You can see the site for further details. There are regional centres and study centres across the country and you can contact the nearest centre for further details.

As mentioned above, you have done 10+ 2 in maths in 1997 that means long time have passed and once again you want to do 10+2 with different subject. At first, I appreciate your stamina that even after passing long period you having interest in study. You can do your 10+ 2 in biology in distance mode through NIOS. NIOS is the abbreviation National Institute of open school. This school is for those student who have failed or discontinued in studies. For getting admission in this school student should have passed 10th from any reputed board. Admission is open through out year. This board is easy to pass as there is no compulsion of subjects like Maths, Science, Hindi, social science etc. There is a biology subject you can take as per choice. Being a correspondence course, they conduct lectures and provide special notes on selected subjects. They provide all kind of help to get the admission in NIOS across the India. They suggest the technique to student how to get maximum marks and can save the year by taking admission in NIOS. Their regional center and study centre spread all over India. As per convenient you can choose study centre. For more information you can check website. Here is the website details:- https://www.nios.ac.in/

Many students after completing their class 10 go for NIOS for clearing the class 12 in the desired subject. You can definitely choose that mode for clearing your class 12 with biology subject.
There is another way to complete class 12 with biology. You can again appear in class 12 exam by taking a single subject biology and you have to apply in your board from where you have done your class 12 for that. That would be a simpler mechanism for getting your class 12 marksheet with biology subject. Go to your school from where you have completed class 12 and find out about this in more details. they may even allow you to attend biology class as a regular student also. In any case you can appear as a private student appearing in exam only in one subject that is biology.

I must appreciate your stamina in the sense you are still trying to make your career with the selection of Biology in your 10+2 stage though you have cleared your class twelve in 1997. This shows how passionate you are for going ahead for the course of Biology.
Though this is not so difficult if you have the time for the persuasion of class twelve Board with the distance course such as IGNOU, Kota Open University, Manipal Distance Education, Utkal University Distance Education and many more.
However, to have the enriched knowledge in Biology without wasting your time in searching out appropriate course materials, you can contact IGNOU officials for the selection of your course taking Biology, Physics, Chemistry, English and Hindi. Follow the course materials in your convenient timing and secure the degree afresh with IGNOU. Moreover any doubt cropping up in your mind be clarified in their contact session organised in the different study - centres. Hence joining IGNOU makes sense while other open universities lack such facilities.
If you desire to pass with the single subject Biology only, talk to the officials so that your request is entertained.