Unable to upgrade from version 5.10.0 to version 8
The following code works for me in Linux Machine:
    cd /usr/local/bin
    mkdir drush-8
    cd drush-8
    composer require drush/drush:8.x
    ln -s /usr/local/bin/drush-8/vendor/bin/drush /usr/local/bin/drush8

Let me explain a little bit:


Go to folder /usr/local/bin   
Create a new folder drush-8  * go
to the folder above  
Install drush (here more info:
https://docs.drush.org/en/master/install/ )  
Finally, create a Symbolic Links that point to new drush version