If it's like the Garmin Legend HCx I just bought a couple weeks ago to replace my old one, try this. Just turn it on and let it get the coordinate where you are standing. Click on the "Mark" and it will come up and show a number for that waypoint. Highlight the number and a box will come up to change it to whatever you want to name it. Then scroll down to the latitude coordinate and highlight it and the box will come up again to change the numbers. Just scroll through and change them to what you want. Then highlight the longitude line and do the same thing to change the numbers to what you want. When you're done with both lines you'll probably have to hit an okay button and then go down to the bottom and hit another okay or possibly a save button. The Garmin manual I got with mine leaves a lot to be desired, but if you have used a GPS before you should be able to figure it out like I did because it should either be like I just explained or very similar.
