Go to the Gear icon
Select Reconcile
Choose the account
Open the History by account or Reconciliation history
Find the reconciliation you want to undo
Select View report
Review the reconciliation details
If available, select Undo
Confirm the undo action
If Undo is not available, manually unreconcile the transactions
Open each reconciled transaction
Remove the R or cleared status
Save the changes
Reconcile the account again if needed