There's a few posts about this; some are just text and others with pictures (links now broken) which may or may or not get updated by the original poster so a new post is in order.
The manual suggests to replace the oil every 30,000 km but I usually do it every second or third oil change (24 0r 36,000 km); or what ever works for you.
You will need a 17mm spanner or socket (better) for both filler and drain plugs, a drip tray, a rag or two, diff oil (EP 80 or better) and something to put the new oil in with; I use a funnel and some clear tubing.
Before you start the oil needs to be warmed up a little so go for a short ride; put the bike on centre stand on level ground when your ready to proceed and remove the RH side case.
Place drip tray next to wheel and undo the drain plug.
While that's happening undo the filler plug -
careful it's plastic and easy to damage. Watchout for the O ring. Sometimes it stays on the drive housing (as in this pic) or comes out as it should with the plug.
When the oil has drained out replace the drain bolt; don't over tighten or you can strip the threads.
Next I grab the funnel and tubing (which could be shorter). With the towbar in place there isn't a lot of room so I route the tube between the bar and muffler. Just need to make sure the end of the tube goes into the filler hole.
Then the oil. Any ep80 or better diff oil will do; I use Castrol. Not sure how old this oil is but it was left over from a diff change over on a previous car.
The correct level (about 150ml ) is reached just as the oil gets level with the bottom of the filler hole. It's not possible to overfill but you can under fill so best to fill until the oil starts to flow onto the hole thread.
I cleaned up the filler plug before putting the O ring back in. Replacing the O ring is suggested in the manual but I'd suggest maybe every 3rd oil change. Remember the plug is plastic so don't over tighten or it will break.
Time to do is about 10 mins if you have everything on hand before you start.
If you keep a log book then take note of the km - cam comes in handy.