Here’s a tutorial you can follow for exporting calendars from your iPhone to a Mac or PC:
Tutorial: How to export iPhone & iPad calendar events
On a PC, iExplorer will export calendars as ICS files, one for each calendar, to your computer which is a universal calendar file format that all calendars app can import and read.
On a Mac, iExplorer will export your calendars directly to the Mac OS X Calendar app on your desktop, and you could then export them as ICS files from it. Just select a calendar in the OS X Calendar app and use the File > Export button.