How do you change the cam chain tensioner on 400ex?

I have a 2003 400ex and hear a ticking coming from the top end. I adjusted the valves but I still hear the ticking. I was looking into getting a manual cam chain tensioner, but I was just wondering what I need to do in order to change the tensioner. Do I need to drain the oil first? Recently got the 400 and don’t have much experience working on it, I’ve mainly worked on 2smokers. Can someone walk me through this? Thanks.