In order to trade a shift, you need to by on your My Calendar page. Select the My Calendar tab at the top navigation and your work calendar is displayed.
Navigate to the Shift you want to trade, by using either the drop-down selector for a week in the future, or utilize the 'Prev' and 'Next' links just above the actual calendar display in the corners. Left-click on the shift you want to trade and a menu appears with actions you can take with the shift.
Select the option "Trade Shift". This will take you to another screen where you will determine what kind of trade you want to make.
From this point, you have a few options for what kind of Trade you'd like to make with your shift. For example: