Sometimes we are curious and do not want to wait a new Emlid image with latest Arducopter version.
In the Wiki there is a way to install a custom version, but it involve creating a new service, not easy for most of us.
Here is a useful way to do it without breaking the Emlid Navio2 services:
First you have to SSH your Navio or enter directly the bash with Keyboard and Screen.
YourComputer:~ yourusernane$ ssh email@example.com
##use nano to create or edit the exec file named install_stable.sh
pi@navio:~ $ sudo nano install_stable.sh
##bin/bash line is to say the file is executed by bash
##point to where is your arducopter file
##rename and overwrite the old file
sudo mv -f arducopter oldcopter
##download the latest stable version
sudo wget http://firmware.ardupilot.org/Copter/stable/navio2/arducopter
##change it as executable
sudo chmod +x arducopter
##end of editing - if you want to install latest dev version, change stable by latest in the script
##and name the file accordingly so you can launch it if needed
Ctrl o to save the file
Ctrl x to exit nano
##you still have to make it an executable file
pi@navio:~ $ sudo chmod +x install_stable.sh
##then run it
pi@navio:~ $ ./install_stable.sh
When done, you can connect with Mission planner and check the version of arducopter (3.5.7 as of Aug 12th).
Next time you need to upgrade (when 3.5.8 is available) just SSH then execute