Do you want to delete everything in the calendar? It is a little unclear if you want to remove and re-import or if you are trying to just remove duplicates. If you want to remove everything and re-import you can do it from the commandline with:
su - zimbra
zmmailbox -z -m firstname.lastname@example.org ef /Calendar
the "ef" will empty folder, which you cannot do to the Calendar under the web client.
If you have the .ics file stored on the mailbox server, you can re-import it from the zmmailbox command with the PutRestUrl command
zmmailbox -z -m email@example.com pru /Calendar /tmp/calendar.ics