If you have an SBI account, you can make use of the netbanking service. The first step is to add your loan account to your internet banking profile. Once you have added the loan account you can view your loan account ledger and continue transactions. To make a prepayment all you need to do is transfer money to the loan account. The  loan account is just like other savings or current bank accounts, it has an account number and IFSC code. If you use SBI net banking you can transfer money and see the outstanding balance go down. 

If you’re not comfortable with netbanking, you can always approach the loan officer at your SBI home branch, to help you with the partial prepayment. You may have to issue a cheque for the partial payment amount. Make sure to have your loan account number handy, as you have to provide it, before you can complete the partial prepayment. 

Volunteering to partially prepay your home loan has several benefits. It helps in reducing the EMI/home loan tenure, thereby reducing your overall loan burden. So, if you have surplus funds in hand - have received an inheritance or a bonus - you can partially prepay your home loan, thereby reducing your home loan burden significantly.