Because of quick breads' firm structure, they can usually be prepared at high altitudes without change, or with only a slight decrease in baking powder.
Using less baking powder or soda usually improves texture and prevents a bitter or alkaline aftertaste. One teaspoon of baking powder or one-half teaspoon of baking soda per cup of flour is usually enough leavening for most quick breads at 5,000 feet elevation.
Quick breads with a cake-like texture are delicately balanced and usually need altitude adjustments. Quick breads not adjusted properly for altitudes have a porous, sugary crust. They also have a coarse, gummy or oily texture and a low volume compared to weight. This can usually be improved by a slight reduction in baking powder and/or soda.
It's generally recommended to reduce baking powder or soda by one-eighth teaspoon per teaspoon called for and reduce sugar and fat by two to four tablespoons for each cup in the recipe.
In the case of quick bread mixes, similar results can be obtained by increasing the flour and liquid by two to four tablespoons per cup in the recipe.
Only repeated experiments with your favorite recipes will give you the most successful proportions of ingredients. Try the smaller adjustments first. This may be all that is needed.
Source: Altitude Adjusters by Karen Kettlewell Harrington, University of Wyoming Cooperative Extension Service (UWCES) publication B-734, 1981, with adaptations from UWCES Cent$ible Nutrition cookbook.