For those suffering the week-rollover problem on the Furuno GP-150. I can confirm it is possible to update the device yourself.
There are a few annoyances. First you need to get hold of a 7-pin cable that connects to the DATA-4 connector. Furuno did not supply that with the unit
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Next you need to use the software that Bill Rouse kindly provided us.
You will need the file with the name "201518-0405_RNMP-20-043(19,200bps)" enclosed with this post (GP150.zip)
Next you will need a USB to serial converter and a D9 plug for the Furuno cable (unless your Windows laptop still has a COM port available).
It connects with only three wires (RD, SD and SG, that is Receive Data, Send Data and Signal Ground)
On the D9 plug these are pin 2 (red wire) 3 (yellow wire) and 5 (blue wire)
Next you need to go into Windows device manager to set some parameters for the USB COM port (make sure the USB-COM cable is connected).
In device manager look up which COM number has been assigned to the cable (under properties), In my case it was COM4:
You need to set some properties as well: set baudrate to 19200 and flow control to "Off"
Now you need to change the file UPe.BAT :
There is a single command inside the Batchfile: "uppg Rload.bin gp100e.bin 0 0 100 0 1 0"
The last one (1) in the command is actually the COM-port number, I had to change it to 4 for my case.
Next follow the manual as enclosed in the ZIP file (GP150 UPDATE.pdf). For me that worked immediately. The update process is fairly slow, make sure you don't run out of power somehow.