Reduce amount for total credits?

how to reduce amount in total suppliers credits?
( credits(5 bill) 10850/- i paid 10500/-)
how to round off the credits?

Supplier credits should only reflect transactions where a supplier gave you money in advance or refunded money (for such things as returned merchandise). The balance is reduced next time you purchase something from that supplier by application of the outstanding credit, so you pay less to settle your bill.

Your example of having bills for 10850 and paying 10500 suggests you may not be using the account correctly. Can you give a more detailed example and show a screen shot of a transaction entry where you used the account?