I reconciled my checking account by mistake, so I deleted it. Now my balance is off. how can I undo this transaction once I deleted it already? This is an online course

Glad to see you in the Community, @rodriguezmo-edua.

When you reconcile transactions in QuickBooks Online, you'll need to undo it if you've reconciled it by mistake. If you accidentally deleted a transaction, you can still recover it using the audit log feature. 

The Audit Log can track deleted transactions. You can then recreate the transaction by viewing the information. 

Once you're done, you can go to your checking account's register and manually reconcile the transaction from there. When you know what you need to change, you can edit the transaction by performing the steps below:

1. Go to the Accounting menu. Then select Chart of Accounts.
2. Find the account and select View register. 
3. Select the transaction you want to unreconcile.
4. Review the check column. If the transaction is reconciled, you’ll see an R.
5. Select the box and keep clicking it until the box is blank. This removes the transaction from the reconciliation.
6. Select Save. Then close your register.

QuickBooks Online Accountant users have this powerful tool in undoing reconciliations for clients. To learn more, you can check this helpful guide: Undo your client’s reconciliations with QuickBooks Online Accountant.

Keep me posted if you have additional questions. I'm here to help.