There are two ways to modify the column order in the current report. The first is from within the Displayed Columns configuration tool. The second is to use the column handle displayed at the top of each column within the report.
From the Displayed Columns Configuration Page
In the Current Columns list on the Displayed Columns configuration tool, the column names can be reordered by dragging the column name up or down within the list. Top-to-bottom in the Current Columns list translates to a left-to-right order in the resulting report.
From the Column Order from the Column Menu
From within a report, columns can be reordered by dragging a column using their handle (highlighted in this image) to the desired location. As you are moving a column in this way, an arrow will appear showing where the column will be inserted.