Thank you for posting here in QuickBooks Community. I'll provide different steps on how to delete imported data once they're not complete.
If these are imported banking transactions, I'd suggest going to the Banking page. From there, you can batch delete them by excluding the entries.
Click the Banking menu. Then, the Banking section.
Go to the For Review tab and checkmark the transactions you want to delete.
Click the Batch actions button and choose Exclude Selected.
However, if the imported data are lists (Suppliers, Customers, Products and Services and Chart of Accounts), you'll need to make them inactive in QuickBooks Online. You can click this article to see the process: How to edit, delete, and restore list elements.
Please refer to this article on how you can reset your QuickBooks account to start over with new books: Delete your data and start over in QuickBooks Online. This process deletes all data entered into the company, and you are no longer able to restore or reverse them.
Let me know if you have any other questions about this or if you need anything else. I'll be around to help. Take care!