To unreconcile one item in the check register:

Line up on the item and hold your Ctrl key and press the letter U on your keyboard.

To unreconcile the account from date to date:

Go to F6 Options, M. Unreconcile from date to date.  This option would be used to unreconcile a prior month for the purpose of doing the reconciling again to correct any errors.  The reconciliation report for the month in question should be available, as there are usually items that had not cleared from the month before. Those items are listed separately on the reports as 'uncleared checks, and uncleared deposits. They would also need to be unreconciled using the Ctrl U function - one item at a time, as they would not be in the date range entered for this option.