Try redownloing the update from the website I'd it will let you then updating again
If you set the satnav to turn off 20 mins after turning the key you can set a timer and keep going out to the car every 15 mins To turn the key on and off so it doesn't shutdown
Also probably worth connecting a battery charger or a second battery via jump leads to insure it's got plenty of power as it takes a very long time to install these updates