If it's not open already, open the archive PST (File>Open>Outlook Data File).
Using either the Calendar or Folder List view of the Navigation Pane, open the
calendar in the archive PST. Display that calendar in a table view like By
Category or Events. Select all the items in the calendar by pressing Ctrl+A
or select the ones you wish to restore. Drag the selection to your default
Calendar folder. All done.