I'm guessing the iMac is currently not syncing with iCloud? Is so, remove all your calendar info from iCloud.com. If your other devices are syncing, that will reflect on them. If they aren't syncing, remove calendar info from them, too. Then, sync the iMac with iCloud by enabling your calendars in the iCloud sys prefs. Then same on other devices.
Always a good idea to have backups created first.
It ended up being a bit more complicated than that because the iMac was connected to iCloud, just not syncing for some reason, so if I disconnected it everything on my iMac would be deleted.
So here's what I did:
- I backed up each individual calendar from my iMac
- disconnected iCloud on my iMac (which deleted everything, but not my backups, of course)
- deleted all the calendars on iCloud (after backing up what I had on iCloud just in case, even though it was the pre-edited data I wanted to overwrite, and which frankly was a pain in the butt, Apple!)
- recreated the same (but now empty) calendars in iCloud. Those empty calendars synced with my iMac.
- I merged the backed up data with the empty iMac calendars
- then this all synced with iCloud and was propagated across all my devices
I have to say MobileMe was much easier, and I had much more control. Apple take note.