You can change your destination inflight two ways. Both use the D button (direct to) on your GPS display. One click will bring up a TYPE:Airport   ID___  NAME___ display. By using the arrows in the diamond in the centre you will also get VOR/NDB/INTERSECTION selection. If you have selected to display all of these using the MENU then they should show on your GPS display allowing you to identify them all. By using the first click option on D and using the arrows in the diamond you can load the identification,click on ENTER and the flight plan will change to that selection.
Second click on D will bring up nearest airfields, move down the list to your selection,click on ENTER.

Round trips are not really possible in flight planner but if you choose two airfields close together you will have a flight path. Making sure you have selected to display NDB/VOR/INTERSECTIONS from the menu bar at the top of the planner,place the cursor on your new flight path and click and hold the mouse button down. You will have captured a point on the flight plan an can move it around the screen. Move it to say a VOR unclick and it will stay there. By manipulating your plan like this you can overfly your start point which as near as you'll get to a round trip. Just use the overfly as your final vector switch off AP and land back where you started.