First, select the waypoint to move. You can do this by using in the usual way, or by selecting the waypoint on the map screen like in the screen on the left and pressing .
On the Waypoint screen select MAP and press .
Now place the cursor over the waypoint, and press so it shows a little MOVE text below it
Now move the pointer to the desired new location of the waypoint and press .
Now the waypoint is at its new location. Ofcourse, you could have entered the new coordinates on the waypoint screen... but you don't always know them (or would have to write them down before you could enter them).