Arranging content in Bb should be a "drag and drop" situation. In the image below you will see that you can “click and drag” an item to relocate its position in the list of items. In the image below, either the Course Menu or the Course Materials can be reordered.

Some people have experienced trouble with this in Internet Explorer but it usually works correctly in Firefox.  However Blackboard has another approach that should also help you out...  In your course content area (Assignments, Course Materials…) should have two arrows (one up and one down) in the upper right of that page (see the arrows circled in red on the image below).  Click on those two arrows and a new box will appear (see the second image below).  This will allow you to highlight an item on the list move it up and down with the arrows to reorder the content on the page as well.

Don’t forget to click on “submit” to apply your changes to the course.