How do i unreconcile a bank statement?

Glad to see you here in the Community, @marmonet. I know some ways on how you can unreconcile a bank statement.

If you're currently using QuickBooks Online Accountant, you can select the Undo reconciliation feature to unreconcile previous transactions. Here's a guide you can browse for the detailed steps: Undo a reconciliation.

If not, you can manually clear the reconciled status by going to the Chart of Accounts. Here's how:

1. Open the Accounting menu.
2. Pick the bank you’re working on, then select View register.
3. Select the Funnel icon to filter the dates, then hit Apply.
4. Find and click the transaction you like to unreconcile, one at a time.
5. Click on the R field until the status changes to a blank (unreconciled) or C (cleared).
6. Once done, click on Save.

See the screenshot below for reference:

Lastly, I'm adding this module on how you can resolve reconciliation issues in QuickBooks: Fix issues for accounts you've reconciled in the past in QuickBooks Online.

It's my pleasure to be of help always. Keep in touch if you have other concerns with banking or QuickBooks in general. Have a great rest of the week!