That is a lock washer which has been levered down onto the wheel bearing nut. What you need to do is use a screwdriver (or something similar) and lever away each segment so that you can get to the actual wheel bearing nut. When you remove the wheel bearing you should be able to check if it is serviceable. Normally you will have to wash off all the grease from it (with either paraffin or petrol) then you will be able to look at it and also spin it.

Your new bearing should come with a new bearing face, which you will need to change (that will require drifting it out of the hub and replacing with new one). Also replace the old grease with new, making sure that the new bearing has plenty of grease on it. Lastly, make sure that when you have replaced the bearing and screwed up the hub nut that you make sure that it is adjusted correctly - do not over-tighten it. Then you need to prise down the tab washer.