It is always nice to have a backup file in case installation goes horribly wrong (very unlikely). Don't forget, this software is provided as is without any warranty.
Install latest version of https://github.com/bkerler/mtkclient.
Perform factory reset on device, take SD card out.
Turn device off. Hold volume down + play buttons and insert USB cable.
Dump whole device to file (~20 minutes for 16Gb model):
mtk rf <path to backup file>Output file will be as big as your device's capacity (16 GB). You can compress it to about 800 MB using zip/rar/whatever and decompress when needed.
Factory reset is needed because your songs won't compress as good as empty space. You may skip it if you don't care about hard drive space.
Make sure you use wf command, which writes to device.
mtk wf <path to backup file>There are reports on this method not working for NW-A40.