Just curious why you say it is out, how have you tested it? Not suggesting you must be wrong, but methodology is important.
I have one of those flash shoe 2way spirit levels, piece of junk, checked it with an engineers level, it was miles out! Much worse than using my judgement!
As for resetting it, no knowledge, just a thought, a reset and initial power on with the camera on a level surface?
I've never checked, but just put my R on my desk, turned it on and the level reads perfect (my desk is quite level). I'd call Canon and ask, but any calibration is likely done by service, and the sensor may be bad too.