I have reconciled an account but used a March date instead of a February date - can I change this not the reconcile has been completed?

Glad to have you here in the Community, @trevor-prior.

You can Undo an entire reconciliation if you're using the Accountant version of QuickBooks Online (QBO) or reach out to your account to do it for you. However, you'll have to manually undo a reconciliation via the register if you have the regular version.

Here's how:

1. Click the Gear icon, then select Chart of Accounts.
2. Find the appropriate bank, then click View register.
3. Select the transaction to edit.
4. Under the Check column, click the box repeatedly to change the status to "Blank". ("C" - Cleared, "R" - Reconciled, and "Blank" - Not cleared or reconciled)
5. Click Save.

Here's an article you can read for more details: Undo and remove transactions from reconciliations in QuickBooks Online.

You might also want to check out this article to assist with the reconcile process: Learn the reconcile workflow in QuickBooks. 

Please let me know if you need further clarification about this, or if there's anything else I can do for you. I'll be standing by for your response. Have a great day.