A small piece of software could be created to “handhold” the user though the process of downloading the update file, format a usb stick and copy the update file to it. Look at the Windows 10 media creation tools as an example of preping a usb stick easily.
Once the stick is prepped and inserted into the car, the car should take over and do the rest. Just inform the user that the update is taking place and prompt them once completed that they can remove the stick (when save to do so if they are already on the move)
Have the car look for update.img in the root of the usb on insertion, validate the signature of the file, validate the file isn’t a downgrade, decrypt, install and delete the file once completed automatically.
Friends and family can also be a source of help to download the file onto a usb stick.