what is the process for applying in UPSC prelims for SBC cast students?

If a candidate is a SBC then one can apply as an OBC candidate if his castes is listed in  the Backward Castes List of Central Government.  Being in Backward Caste list of State Government makes you eligible only for State Reservation and not for Central Government Reservation. Apart from it you should also make sure that you satisfy the conditions of wealth test. Wealth criteria is used to determine Creamy Layer Status within the Other Backward Class (OBC).

So if your SBC means State Backward Caste then you should check the central government list to find out whether your caste is listed or not. If it means Scheduled Backward Caste then also you need to check the list in order to verify whether you belong to Backward caste under the State list or the Central List.