2004 sportsman 400 bogs down???

I have a 2004 sportsman 400 that bogs down when I go up hills.  I have taken it to the dealer ship before and They did fix it.  That was a year and a half ago.  It is starting to do it again so whatever they did didn't really fix it and I don't want to take it back to them because I feel they are jerking me around on this because I'm younger.  I have read online and other people seem to have this problem as well. Some people say that it needs to be rejetted.
Is this true? and how do you rejet your carburetor if that is the problem?  

I am riding between 1,000 and 3,000 feet usually