It's easy, just drain the old stuff, flush it out if you want, then fill it back up with about a 50/50 to 60/40 mix. Let it run with the cap off making sure to rev it up every so often. This will help the coolant flow through the system and get any air out of it. When you feel all of the air is out of the system, put the cap back on and make sure your overflow reservoir is filled to the proper level. Double check the levels before and after your next ride.
Are you changing the coolant just as a maintenance issue or are you having overheating problems?
If you are having overheating problems, check to make sure your radiator is clean and no mud/dirt is built up in between the fins.