MY2012 and later satnav maps can be upgraded without involving a dealer - at least on NAS LR4s - with a USB stick purchased online (there is a VIN-dependent activation code that you need to enter with the touchscreen when you start the installation, to insure the stick will work only on your vehicle. The code is generated when you order the USB stick and enter your VIN.)
Going back to the original question, just because the upgrade process was interrupted when you turned off the engine, it dioesn't mean it has failed. The process automatically resumes the next time you start the engine, for as many on/off cycles as necessary, until it is complete. I don't recall installation taking 3 hours overall - it took maybe an hour in my 2013 the last time I upgraded the satnav about 3/4 years ago - but it does take quite a bit of time, and what I wouldn't do at this point is attempting to reset the system with a battery disconnect... There is a 50/50 chance it may work
or permanently brick the hard drive - which would be a huge headache as it's not like you or a dealer can reformat these proprietary drives as you would on a regular computer if you screw up.
If I were you, I would insert the stick back into the USB port under the arm rest, restart the car and hope and pray the upgrade process eventually resumes where it left off when you removed it, because the instructions that came with the stick are quite clear and warn you not to remove the USB device until install is complete:
https://landrover.navigation.com/pr...2018-NA/sku/T1000-27160/en_US/LandRoverNA/USD