Welcome to the Community. It's important to correctly record your transactions in order to keep your books balanced. QuickBooks Online is a great program that allows you to simply track your sales and expenses. I'd be glad to provide you with some information on how to delete your old data in QuickBooks.
QuickBooks Online offers the ability to delete data and start over for accounts less than 90 days old. Purge company is the process of deleting any information entered in QuickBooks Online. I encourage you to check out this article on how to delete your data and start over in QuickBooks Online.
If your account is over 90 days old, I encourage you to delete the data manually.
If you'd like to include the option to delete account data older than 90 days, I'd recommend that you send us Feedback by clicking on the Gear icon at the top. This is sent to our Development Team to consider for future updates.
Feel free to reach out if you have any questions, I'll be happy to help.