I have a friend who moved to Colorado. She is not a decorator or anything but I told her I would ask the forum for some help what kinds of adjustments do you have to make to get decent cakes from mixes or scratch brownies? She is frustrated...again she is not really a baker, but needs some advice!
It's tough to use mixes at high altitude because you can't control the leavening agents, but one thing she can try is add about 2-4 Tablespoons of flour and 2-4 Tablespoons of liquid.
When I lived in South Africa (about the same altitude as Colorado), I would add 2 Tablespoons of flour per every cup, reduce 1/8th teaspoon baking powder per cup of flour, reduce sugar 2 Tablespoons per cup of sugar and increase liquid 2 Tablespoons for every cup of flour.
When I lived at just under 9k ft in CA I found I needed to add more liquid and bake longer for a cake mix. I don't remember now exactly how much but maybe that will help. I made no other adjustments like the other poster said.