The files at each level of your project's hierarchy display in the order in which you added them to the Builder. This is also the same order in which the files will be built. You can rearrange the order in which the files are displayed to change the build order, and also to help you visually understand your hierarchy. For example, you may want to rearrange the files so the source files that belong to a build file appear before the subprojects of that build file.
Alternatively, after you highlight the files you want to move, press Ctrl+Up or Ctrl+Down to move the files up or down in the list.