I accidentally set up a bank account instead of creating a category and have entered several transactions. How do I fix it?

Go to your Accounting tab and then Chart of Accounts.  Find the account in the list and then click on edit.  Change the account type from banking to whatever is is you need then save.  This will automatically update with the transactions already assigned to it.

I know a way to help you change the bank account to a correct category, @akdillon7651. This way, the transactions will reflect or use the said category going forward.

If you're referring to the chart of account categories, I agree with Anjunadeeper that you'll need to go to the Accounting tab to modify it. Let me give the detailed steps: 

1. Go to the Accounting tab on the left and select Chart of Accounts.
2. Locate the incorrect account you've created. 
3. Click the small drop-down arrow under Action and pick Edit. 
4. Choose the appropriate options above and modify the Save account under a field ( if you used the new account panel). 
5. Update the necessary details and hit Save.

For complete procedure, check out this article: Add an account to your chart of accounts in QuickBooks Online.

However, if you're referring to Item Categories, you can set it up by following the steps below: 

1. Go to the Sales tab, then select Products and services.
2. From the more More drop-down, pick Manage categories. 
3. Click the New category tab and enter a name. 
4. Hit Save. 

After that, add the uncategorized item to the category you've created. This way, the transactions will appear automatically under that category. For complete steps, click this link: Group your products and services into different categories. 

Once done, you'll need to inactivate the bank account you've created. Click this article for complete instructions: Make an account inactive on your chart of accounts in QuickBooks Online.

Furthermore, visit these articles below on how to modify item type for a product or service and manage your register in QuickBooks: 

• Change product and service item types in QuickBooks Online
• Learn about the chart of accounts in QuickBooks Online

I'm always here if you need more help in managing your transaction or anything else. Just let me know by leaving a Reply below. Keep safe, and have a great rest of the day!