How can I make an account as a sub account of another major account after that account created in COA already

Hi there, kgugagan-gmail-c.

Yes, you can turn a parent account to a sub-account through the COA page.

Here's how:

1. Click the Gear icon. Then, choose Chart of Accounts.
2. Choose the main/parent account and click the drop-down arrow under the ACTION column.
3. Select Edit.
4. Checkmark the Is sub-account box. Then, enter the parent account.
5. You can make any changes to other necessary fields.
6. Hit Save and close.

For additional information, you can click this article: Create subaccounts in your chart of accounts in QuickBooks Online. This also provides details about other things you can do to keep your accounts in order.

Please check this article to see steps on how you can organise your list of accounts, customers, and suppliers in QuickBooks Online: How to merge accounts, customers, and suppliers.

Please know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.