There is a link at the bottom of the League Administration tools section "Edit Head-to-Head schedule". Once accessed the current weekly schedule is displayed. Simply click on the "Edit" Link for the week to be modified.
Click on the checkboxes next to the teams to be switched. You will also be able to assign a "Bye" week instead.
Click on "Save Changes" to execute the change.
NOTE: Changing a matchup may create an imbalance in the numbers of times divisional rivals face each other. It is a good idea to review the entire schedule when making changes to ensure changing one week doesn't create an issue later in the season.