Based on messages from a few years ago it appears that the only way to update ST70's or other Raymarine SeaTalkNG gear was to use a Raymarine plotter or remove the item(s) and return them to a Raymarine service facility or dealer.
There was talk at the time of an imminent Raymarine magic usb dongle that would solve the problem.
Whats the current state of play? Can a Raymarine ST70 system or an SPX AP be field updated without removing them from the boat if a Raymarine plotter is not installed?