If you matched multiple QuickBooks transactions to a single downloaded amount, you won't be able to undo the match directly from the QuickBooks transactions themselves.
Instead, you undo it from the In QuickBooks tab of the Banking page.
To find your transaction more easily, click a column header to sort up or down by amount, date, description, and so on.
The item moves back to the New Transactions tab. The multiple QuickBooks transactions can then be matched again.