On my "basic" yellow Garmin etrex:
To mark:
Go to 'mark' scroll down to the coords and click the 'enter' button
Then click the scroll buttons to highlight what you want to change, and click the 'enter' button to indicate you want to change it.
To move the 'cursor' i just use the up and down keys on the left side
On my Garmin Legend cx:
Basically the same thing, my cx has the 'knob' on the front to use to change things, up, down, left, right to scroll, push it in to enter
If you still can't figure it out...take it into a store that sells them and just say "i have this, and I can't figure it out for anything" around here, the Great Outdoor Provision Co usually has at least one knowledgable staff person, or they can pull out a manual and help you figure it out.
Good luck!