How to unmatch transaction

Thanks for posting here again, @rchiarchi,

When you want to change something on a matched bank transaction, edit the status of the entry. Here's how:

1. Click the Gear icon.
2. Tap Chart of Accounts.
3. Find the account you want to review.
4. From the Action column, click Account history.
5. Locate the entry from the list that shows a code C (Cleared).
   
6. Click the field multiple times to change the cleared status. A Blank status means the entry is not cleared or reconciled.
7. Once done press Save. 

See this link for the steps: Undo or remove transactions from reconciliations in QuickBooks Online

I want to make sure this is resolved and I'll be right here if you need further help. Have a lovely day!