We have created a bank ledger without mentioning opening balance.Later we tried to edit ledger opening bal under chart of a/c's but there is no such option as in Tally.

Dear Sir, 

Opening balance cannot be edited once entries are passed. Now you will have to pass a Journal entry on the respective date .

JV would be as 

Bank Dr

To Opening balance Equity (You can check exact name in Trial balance / Balance sheet)

Let me know if any issues. 

CA Alok Shah