You can download the latest Garmin (cGPSmapper) from this link. http://www.malfreemaps.com/viewforum.php?f=46
Not sure about the directory structure in your Nuvi 42LM.
Assuming you have an SD card installed in the card socket for your Garmin, simply do this:
1. Connect your Garmin to your computer via a USB cable.
2. Put your Garmin in USB Mass Storage mode.
3. The Garmin should show up as a removable disc (like a USB stick) in the usual place.
4. If there isn't a folder on the Garmin called 'Garmin', create one.
5. Into this folder, copy the .img file that you downloaded.
6. Rename it to anyfilename.img. (rename the file to any name
as you wish)
7. Eject the Garmin as you would a USB stick.
8. You should see the new map in your device.
Alternatively, you can try to download the updated map from Garmin at this link:https://www.garmin.com.sg/mapupdate/
I hope this helps.