How do I delete the cash sub account of my checking account?

Glad to have you join the Community, httmllc. 
 

Let me walk you through some steps on how you can delete a sub account and edit an account type in QuickBooks Online (QBO). Here's how:
 

1. Go to Settings.
2. Select Chart of accounts.
3. Locate the sub-account you want to delete.
4. Click on the drop-down menu in the Action column.
5. Select Make Inactive.
   
   

Moreover, to make all entries appear as checking account, you can follow these steps:
 

1. Go to Settings.
2. Select Chart of accounts.
3. Locate the account you want to change.
4. Click on the drop-down menu in the Action column.
5. Change the account type to a checking account.
6. Click Save.
   
   

In addition, if you want to learn more on how to manage reports in QuickBooks, you can refer to this article:  Run reports in QuickBooks Online

Your thoughts and questions are always welcome. Feel free to comment below if you have questions or clarifications about managing sub-accounts or any other QBO concerns. We're here to provide support 24/7. Have a good one!

@httmllc-gmail-co 

The advice given by @ShangY is incomplete.  When you make the cash subaccount inactive, QB zeroes out the balance in the account and also reduces the balance in Opening Balance Equity (OBE) by a corresponding amount to keep your balance sheet balanced.  You will need to move that balance from OBE back to your checking account by making a transfer (New > Transfer).  Transfer the balance from OBE to your checking account and you should be all set.