Note that opening a previously closed period may result in new transactions added to the period. New transactions will create new journals that must be managed and imported as applicable for your specific case.
To open a closed period go to Periods and select the latest closed period by clicking on the line. Click the three black edit dots in the upper right corner and select 'Open period'
It is only possible
Don't forget to close the period again after making any required changes. Also not to forget to get the new journal data if transactions have been added through invoicing or payments.