You must have some basic Linux knowledge before performing the operation.
You connect via SSH to your navio
YourComputer:~ yourusernane$ ssh pi@navio.local
enter password
##point to where is your ardurover file
cd /opt/ardupilot/navio2/ardurover-3.4/bin
##List the files in the folder
ls -l
##rename and overwrite the old file
sudo mv -f ardurover ardurover342
##download the latest stable version
sudo wget http://firmware.ardupilot.org/Rover/stable-3.5.1/navio2/ardurover
##change it as executable
sudo chmod +x ardurover
##reboot
sudo reboot