I had the same problem when I first got mine.
Page to the menu with mark, waypoint etc on it
Select mark
On the next screen OK should be highlighted,
Use the down button to move the highlight down to the co-ordinates at the bottom
Press enter then use up and down to navigate through the coordinates and enter to select the number/ letter you wish to change, then use up and down to select the character you want to insert, enter again will take you back to the edit location screen.
Once you have changed the location select OK at the bottom and it'll take you back to the MARK WAYPOINT screen.
Highlight OK and it'll record it in your waypoints.
You can also edit the waypoint name by highlighting the number above OK and changing the characters using the same method as above