Hello,I will be glad to try and assist you with this,Welcome to Just Answer.
O.k.,first you have to make sure you have the cable that came with the device when you purchased it,this cable connects this device to your computer,then you will have to log into Garmin's website(I have posted the link here for you)and follow the instructions for your device on updating the maps.
The website should walk you through the rest to setup and update your device,there is a free map update link to click on on the website,you will have to click on it and follow the instructions to see if your device qualifies.
Since you do not have the correct cable for your device you will have to purchase the correct cable either online through places like Ebay or Garmin or similar.
You will have to have the correct cable to do this update to prevent any damage to your GPS device.You may have to do some searching to find the correct cable.