Actually, had to do this on an old Owncloud instance where I was not able to export the calendar data to be imported into a new NextCloud instance.

1. On Old Server
   1. Log into the database for OwnCloud
   2. Find the calendarid of the user in question (don't remember query)
   3. exit database
   4. execute following command<code bash>mysql owncloud -e 'select calendardata from oc_clndr_objects where calendarid = 32' > calendar32.ics
   5. copy to workstation
2. On Workstation
   1. Open with geany
      1. search and replace all '\n' with nothing. If you feel brave, replace with newlines
      2. convert to Linux line endings
      3. save
   2. Open Thunderbird
      1. Go to calendar
      2. File | Open, find ics file
      3. Export Calendar
   3. Open Nextcloud web interface
      1. Calendars
      2. Import