I reconciled my bank account as of 12/31/2020 but I used 12/31/2021 in the system in error. How can I delete the completed rec and start over?

I can definitely help you with that, rick83.

If you've reconciled an account by mistake, you can undo the reconciliation and completely start over. Before diving in, please take note that you can only unreconcile transactions one at a time. If you need to undo the entire reconciliation, I'd recommend inviting your accountant to your company and let them undo the reconciliation period at once.

When you're ready, here's how to unreconcile a transaction:
 

1. Select Accounting from the sidebar menu.
2. Go to Chart of Accounts.
3. Find the account holding the transaction, then click View register from the Action column. 
4. Select the transaction you want to unreconcile to expand the view.
5. Review the check column. You’ll see an R if the transaction is reconciled,
6. To remove the transaction from the reconciliation, keep clicking the checkbox until it becomes blank.
7. Click Save to complete the process.

Once done, you may now reconcile your accounts again to ensure that they always match your bank or credit card statements.

Additionally, I encourage reading these articles to help resolve common reconciliation errors and issues in QuickBooks Online:

• Fix issues at the end of a reconciliation
• Fix issues for accounts you've reconciled in the past

Let me know if you need more help in managing your bank accounts by dropping a comment below. I'll be more than happy to assist you again.

Try going back in to reconciliation and changing the date to 12/31/20. Put in the ending balance and reconcile again. You will not need to mark anything.  You might get a warning about the date, but I believe it will work.