Hello Ramesh, 

Ita not a very big issue. You can easily change your category from general to obc. 

What you need to do is that, first you have to have an affidavit mentioning about this issue and the changes you want to make. Also you will need you original caste certificate to verify the same.

So you can produce both these documents whenever asked or in the next round. 

Hope this helps, Thank You 

All the best