Once you reconcile a period, the system locks down that period and all periods before it, as reconciliations are a history-to-date report.
In order to add, edit, or delete records in a reconciled period (or prior to a reconciled period), you must unmark that period as being reconciled.
How to unmark a period as being reconciled
From the Reports tab > All Reports > select 'Periods' from the drop down list and click 'Run'.
Then select the period to be unmarked and click on the Report Submenu 'Actions' > 'Unmark Period as Reconciled'
Remember: All periods prior to (earlier) than a reconciled period are considered reconciled as well.
Reconciling a Period
Tip: Did you have to unmark several periods?
To mark them as reconciled, start with the earliest one that was reconciled, and run a Reconciliation Report. This will mark the period as reconciled.
Then run a report for each subsequent period until you have finished.