Thank you for dropping by here in the Community. I'm here to help provide information about the payment in QuickBooks Desktop.
Right now, unmatching or undoing an assigned payment from the Bank Feeds is currently unavailable. However, you can restore an existing backup copy prior to assigning of the transactions or by manually editing them in the Desktop version.
In case you need the steps, here's how you can restore a backup:
Click the File tab at the top menu bar.
Click Open or Restore Company.
Select Restore a backup copy.
Choose Local backup or Online backup.
For future reference, I'm adding the support link that I recommend on this: