In Ubuntu Apply the following command CTRL+ALT+T [TO OPEN TERMINAL] Or Go to Applications > Accessories > Terminal.
Just Follow This Step to updating koha 19.05
STEP 1: sudo apt-get update
STEP 2: sudo apt-get upgrade
STEP 3: echo 'deb http://debian.koha-community.org/koha 19.05 main' | sudo tee /etc/apt/sources.list.d/koha.list
STEP 4: sudo apt-get install koha-common
STEP 5: sudo apt-get clean
STEP 6: sudo koha-upgrade-schema library
[Choose your instance name library to your instance name]
STEP 7: sudo koha-rebuild-zebra -v -f library
After Completing this zebra restart, now refresh your koha about page there you can see the koha version 19.05

Thanks & Regards
Chitra
Librarian,
Chettinad Vidyashram,Chennai.