atv rear end flush/fill

I have a 2005 trx250 (recon). Rear end oil is kinda dirty so I was wondering how to go about flushing out and filling it. I've got the oil (80w90), and I assume there's a drain plug somewhere...would I just pour in the oil and let it run through until the clean oil comes out the drain hole? When that part is done, how do I know how much oil to put back in, I don't see a full line. I was told it doesn't hold much (less than 3 ounces or something). From the photo can anyone tell me which is the drain plug/screw?http://i.imgur.com/rbw2JEnl.jpg

well i did some searching on youtube and found the drain plug. I let it drain for a good while then poured in new oil and saw some more dirty oil run out. finally when the fresh oil started to pour out I put the plug back in and filled it up til it hit the overflow hole. hope I did it right!