Need advice/help updating firmware & Maps on Nuvi 1390 on my Chromebook OS ... I've got latest maps on a mini SD

hi, I  am not very tech savvy, I just started using a Chromebook Asus...and Linux is 110% over my head.. 

With that said... I looked through the site and some threads and could not find an ACTUAL link for downloading Garmin Express and I can use any help I can get... thanks in advance!! 

oh... and I do have one of the most updated versions of the North America Map on a SD card that is plugged in to my computer right now but i cannot connect the dots!