Updating
It's assumed that Bookstack was installed by the Ubuntu script.
- Very important, as always backup your Bookstack instance
 - by default Bookstack is installed under 
/var/www/bookstack, if that wasn't changed, then go there - try to pull the latest changes: 
git pull origin release - if that fails because there are 
updatesupdated files in the directory, run:git config core.fileMode false - then: 
composer install --no-dev - last one: 
php artisan migrate 
Also don't forget to clear cache:
php artisan cache:clear
php artisan config:clear
php artisan view:clear
 
Sources: