Can someone tell me if I can downloat a map on my etrex 20 and i don't like it can I delete it? How?
This should be covered in your owners' manual.
If you connect your Etrex20 to a computer with a USB cable it will show up as a drive. First copy the contents of your Etrex to the computer. Then in the \Garmin folder you will see a number of IMG files. You can delete any that YOU have added. Check the dates. Don't delete ones you haven't added. IMG files in my \Garmin folder on my Etrex30 are gmapbmap.img and gmaptz.img. You don't want to delete them.
Also, nearly all maps on this site need to be installed in BaseCamp before they can be put on your Etrex. If you have trouble getting the maps into BaseCamp or onto your Etrex, be sure to read the tutorials linked at the top of this page before you ask for help. It will make your questions much better informed.
BaseCamp execute program MapInstall to send maps to device. MapInstall is a separate program, that comes with BaseCamp installation but can be downloaded and installed alone too.
Connect GPS to PC and run MapInstall. On first screen select your eTrex in device list and click "Customize...". Then you will see a list of all maps in PC and in GPS with option to send maps to GPS and/or to remove them form GPS. Factor installed maps are shown too, but without option to remove.