...my Legend was preset to WGS 84 from the factory, so hopefully you shouldn't have to set anything. But...
With your unit on, hit the Page button (top one on the right) until you get to the Main Menu (the screen with all the icons.)
Use the ClickStick to select Setup and on the next screen, select Units.
The second option down is Map Datum. Click on this, and from the pop-up menu, find WGS 84. Select it, and you're all set!