Hi,
Without the correct tool, there are a couple of bodges I have used!
You could use a small flat head screwdriver; angle it into the notch and give it some gentle taps with a hammer, go easy and stop immediately if it looks like you are damaging the notch.
Also lay some protection around the taps in case your screwdriver slips.
A couple of squirts of WD-40 left to soak, may make the removal easier.
Alternatively, if you have a normal cutlery knife (not a sharp kitchen knife!) that you aren't fond of, It may be able to be bent around the shaft with the back (straight) edge sitting in both notches.
Then use the handle as a lever to gently turn the retaining ring (anticlockwise!).
As I said, these are bodges, so try at your own risk!
Others probably have better ideas!