First download USB Garmin drivers,connect nuvi USB ,set Windows to see hidden files and folders, go to .System folder in nuvi and backup on pc your preloaded files: gmapprom.img gmapprom.unl and gmapprom.gma.
Then delete the old map on Nuvi along old .unl and .gma.
Unlock the new map, rename it to gmapprom.img and put it in the same place where you found the old one. Safely disconnect Nuvi from pc.