Under the chaincase, you'll find a drain hole in the bellypan. There is a drainplug in the bottom of the chaincase that lines up with that hole. Take the allen head plug out of the chaincase and drain the oil. Fill the chaincase through the dipstick hole. I have a funnel with about a foot of tubing on the end so I don't have to remove my muffler.
If you want to use the factory recommendation, go to your dealer and spend 12 bucks for Polaris chaincase oil. I have been using Automatic Transmission Fluid in all of my sleds for several years now. It has very similar properties to the expensive chaincase oil for a fraction of the price.