How to change the caste from that of one parent to another

A person who was born to the parents of different castes can opt for any one of the two castes. As you want to go for your mother's caste she has to give a declaration stating that you were brought up by her and followed the norms of her caste while bringing you up. With this declaration, you have to approach the Revenue department of your area. They will give the guidance as required. Regarding the declaration from your mother. you have to contact any good lawyer in your area and he will help you.
In your case, you are opting for an OBC caste which will make you lose the reservation facilities and other facilities. Are you prepared for that? Then you can go ahead and contact a lawyer.

There would not be any objection from any side if you opt for mother's cast to be retained for all your official purpose. You need to obtain a declaration from your mother that she is involved for bringing up you meeting all your financial expenses apart from looking after all your activities. This statement has to be certified by the Mukhia or Gram Pradhan of the village and in case of living in a town area, you need to contact S.D.O regarding the authenticity of such a declaration.
Once this formality is over, approach the lawyer of your local court to get issue resolved. An affidavit would be required for the change over of your caste. With the guidance of the lawyer, you can have an affidavit certificate indicating your caste status in the general category. Get the same advertised in two papers - one in local and the other in prominent English Newspaper and preserve a cutting of such a notification for the production of the same to be shown to the verifying authorities in case of need.
However, before executing the same for your benefit, you should examine the pros and cons of the benefits which you might have got by retaining the caste status of your father belonging to ST. While seeking for an employment in the job both in the central government undertaking or in the private, you would enjoy the concession in terms of marks, age and other benefits prescribed for a job for an aspirant of ST. You would lack such facilities being a candidate of general category.

If someone has parents from different castes then one can choose the cast of either father or mother or if one is having one's father's caste so far then one can definitely change it to one's mother's cast. It is to be noted that in doing so one has to forego any cast related benefits that he would had taken from the father's cast. This is an important point and is to be kept in mind.
Now as regards to changing of caste from father's caste to mother's caste one has to follow the laid down procedure in the matter. Generally these things are dealt in the SDM or ADM or DM or revenue office in the district or Taluka. One has to apply for the change of caste in the prescribed format by giving all the details about parents and their castes and option for changing the caste now to mother's caste. Necessary declarations are to be made so that no misuse of the regulations and facility could be attempted by an individual. It would be better to take the help of a lawyer in the matter. This will also require you to publish this change in local newspapers and get the change registered in the gazette.
One thing which is important in this matter is that the decisions of changing caste are to be taken after giving a good thought to the option so one should consider all the pros and cons of the matter and then only go for the procedure.