So you still have the 3.8.1 arduplane version.
Run the following commands
pi@navio:~ $ cd /opt/ardupilot/navio2/arduplane-3.8/bin
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ ls -l
total 1224
-rwxrwxr-x 1 root root 1252364 Sep 14 12:31 arduplane
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ sudo mv arduplane arduplane.old
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ ls -l
total 1224
-rwxrwxr-x 1 root root 1252364 Sep 14 12:31 arduplane.old
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ sudo wget http://firmware.eu.ardupilot.org/Plane/stable/navio2/arduplane
–2018-01-26 17:11:56-- http://firmware.eu.ardupilot.org/Plane/stable/navio2/arduplane
Resolving firmware.eu.ardupilot.org (firmware.eu.ardupilot.org)… 208.68.38.241
Connecting to firmware.eu.ardupilot.org (firmware.eu.ardupilot.org)|208.68.38.241|:80… connected.
HTTP request sent, awaiting response… 200 OK
Length: 1262084 (1.2M)
Saving to: ‘arduplane’
arduplane 100%[================================================================>] 1.20M 422KB/s in 2.9s
2018-01-26 17:12:00 (422 KB/s) - ‘arduplane’ saved [1262084/1262084]
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ ls -l
total 2460
-rw-r–r-- 1 root root 1262084 Jan 8 23:48 arduplane
-rwxrwxr-x 1 root root 1252364 Sep 14 12:31 arduplane.old
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ sudo chmod +x arduplane
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ ls -l
total 2460
-rwxr-xr-x 1 root root 1262084 Jan 8 23:48 arduplane
-rwxrwxr-x 1 root root 1252364 Sep 14 12:31 arduplane.old
pi@navio:/opt/ardupilot/navio2/arduplane-3.8/bin $ sudo reboot
Commands are in Bold
After each command, you can compare your result with mine (I just did it).
Some like ls -l are just to check the result…
Marc