Tools > System Utilities > Admin Utilities > Finance Utilites>| Unpost Check Reconciliation is the tool to use to unpost the month.
The process is changing the closed box on the transactions from a Y to an N. Thus it will be necessary to run the tool for each date you used in the reconciliation process.
If you used the final date of the month for all of the transactions you will only need to run the tool once. To determine which dates to use, run the check transaction listing found on the process tab and select on the date range for the month to be re-opened and review the reconciliation date column in the resulting report. Run the tool for each date shown.
The unpost process does not reinstate the prior ending bank balance. It will be necessary to enter the correct balance before running the reconciliation report.