Changing account structure

I created my bank account under assets in chart of accounts and made entries using journals.

Now, how can I change that account to appear under bank account and containing the entries.

You can’t. You need to create bank account under Bank accounts tab directly. What you have created is plain general ledger account and it’s currently not possible to convert general ledger account into bank account.

So just create new bank account, then go back to those journal entries which were posting to general ledger account and change those journal entries so they are posted to special Cash at bank account once your bank account is created. Then delete general ledger account you’ve created.