Please how do i enter bank opening balance for an already set up bank account in chart of account?

Welcome to the Community, @davig. 

I've got the steps to help you enter a new opening bank balance in QuickBooks.

You can use a journal entry to add an opening balance to your active account for accurate financial records. Here's how:

1. Sign in to your account.
2. Go to the +New icon, then Journal Entry.
3. Enter a date that comes before the oldest transaction in the account. It'll be the opening balance date.
4. On the first row of the Account column, select the dropdown menu to choose the account you want to enter the opening balance for (Asset Account).
5. Add a note in the Description field so you know what the journal entry is for.
6. Choose Opening Balance Equity on the second row of the Account column.
7. Use your bank statements to enter the balance of your real-life account on the date you picked as the opening balance. You'll need to enter both a debit and a credit to keep things accurate. Enter the debits and credits based on the type of account you entered on the first row.
8. Save the transaction when done.

See the screenshot below for your visual reference. 

Moreover, if you haven't reconciled the account yet, you'll need to reconcile the journal entry to prevent it from affecting future reconciliations:

1. Go to Transactions and select Chart of Accounts.
2. Find the account, then choose Account History from the Action column.
3. Locate the journal entry you created and select it to expand the view.
4. Select the box in the tick column until you see an R, which reconciles the journal entry.
5. Click Save.

For more tips about entering an opening balance after creating an account, you may open this article: What to do if you didn't enter an opening balance in QuickBooks Online.

On the other hand, I'm adding this material for more tips while entering and managing an opening balance for your bank, credit card, and other types of accounts in our system: Enter and manage opening balances in QuickBooks Online.

If you have other questions while setting up and working with your chart of accounts, let me know by leaving a comment below. I'm just a few clicks away to help you again. Keep safe!