I'm sure others may disagree with me, but this is not an "Easy job" . Depending on whether your work area is heated or not, if you have allot of experience doing this type of thing, and how familiar you are with some basic tricks all factor into how long it will take you to do it and whether you are better off to take it to your dealer. There are many excellent step-by-step write ups of different ways to do this on the TOTALYAMAHA.com site - which you can browse or join free.
Some would suggest to replace them by just loosening the track and forcing the old ones out thru the track windows but you might want to consider dropping the suspension and doing it that way since you have some miles on the sled and it is generally a good idea to check over all the other suspension components at that kind of mileage. I have done it both ways and would probably suggest you not opt to remove the suspension unless you feel comfortable doing something fairly major.
Whichever way you do it, the critical step that everyone has problems with is the #3 phillips screw that holds the hyfax (at the front of the rail on each side). Use the proper size screwdriver, held at the proper angle to avoid damaging the screw head!! These screws have blue Loctite on them from the factory and may come out hard. After loosening them a few turns, tighten them a few turn and repeat this as you remove them. This will help to avoid stripping the threads and the heads. Also be careful when putting them back that they are not crossed.
Good Luck whicever method you decide to use to replace them.
PS Rember that the single biggest factor in premature wear (other than snow conditions) is the track tension. Over the past 25 years, I have found that the Yamaha spec is a bit tight and instead go by seeing the track "hang" about 3/4 - 1" from the hyax to the clips when the sled is off the ground.
Slow Down + STAY RIGHT!!
Remember, ALL snowmobile accidents are preventable.