You can undo an allocated bank transaction in a few quick steps;
Step 1 - Select the 'Account Transactions' tab
From the Bank Feeds dashboard select the account and the 'Reconcile Items' button. Then, from the reconcile page select 'Account Transactions' .
Step 2 - Find & Redo
Use the search bar to find a transaction. Once found, select the 'Redo all' button to undo the whole allocation.
The bank transaction will then return to the 'Reconcile' tab so it can be reconciled again.