installing new map ..?

[HondaMechanic]

I bought a used Garmin Legend which came with a map ..The map isn't very detailed ..I also have the computer cable that came with it.. I don't have ant CD's for it .. I am buying a better map CD of my area .. Can I save the map that is on the GPS unit now .. Or will I lose it ..?..

Or would it be better to make a map with a topo map of my area , if so what would I need ..?

I'm new to GPS's so any help would be great ..

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