Because you applied for the jee mains as a general category candidate, you should have applied as an obc applicant to claim your reservation, as the cut off would be lower than for a general applicant.

However, if you do not modify your application form between the 19th and 21st of January, you will not be eligible to modify your category.

Dear aspirant,

As you applied your jee mains as a general category candidate and considering you are an obc candidate you should have applied as an obc candidate to claim your reservation as the cut off would be low as compared to a general candidate.

But as from 19th Jan -21st of Jan you haven't edit your application form then you won't be able to change your category.

For making obc candidate you have to make income certificate and any candidate can make this.

I don't think you can submit any undertaking now.