Its probably a bit strange, but to update an existing item (unless I have a bug), is you have to load the file you previously sent and edit that. as it gets the online id from that.
So click on create, load the level file you sent (even if the editor is still showing what you want).
edit
share, but don't change or edit the name.
If that doesn't work let me know and I'll check it out again.

p.s. sorry I didn't spot this before and no idea why I didnt see it...
Oh its in workshop discussions, didn't even notice there was a sort of general workshop discussions... oops, I will also check here in the future.

Thanks Steve! I will try that next time I update my level. I wasn't loading the file first, so it will probably be just that.