At altitude there's less oxygen so even if your fuel:air mixture is correct you will never have as much as much power as you will at sea-level, that's just something you have to live with unfortunately. If you think you are running too rich becasue of the altitude (sooty plug, misfire under load) you can reduce the amount of petrol in the mixture in two ways: from about 1/4 throttle to about 3/4 open the mixture is mostly controlled by the needle and needle jet in the carb'. The needle is normally held in place by a little spring clip sitting in a groove, but ther are normally several grooves. By lowering the needle (putting the clip in a higher groove) you will weaken the mixture, by raising it you will richen it. Above about 3/4 throttle the main jet controls the amount of fuel. The ony way of altering this part of the carburation is to change the jet for one with a different size hole (big hole = more juice = richer, small hole = less juice = weaker). All jets are stamped with a number which corresponds to the size of the hole but obviously you need a supply of main jets first...
However, unless you are really sure you know what you are doing, or the bike starts to seriously misfire due to being over-rich at altitude. I'd leave everything alone and accept that altitide will rob your power until you get lower. If you mess with the carb' and run too lean you will also run hot and risk damage by overheating or promote detonation which will seriously spoil the party