Long nosed pillars to remove the split pin - rotate the hub until a small hole in the hub aligns. You may need to remove some grease to fin the hole. Socket over the nut and see if you can tighten it - DO NOT OVER TIGHTEN as you will make it harder for the bearings, they then may seize and fail.
Then refit the split pin (ideally with a new one). This is the hardest (most fiddly) part.
I'd remove the brake caliper and disc and have a look at the condition of the outer bearing while you're at it.
