Upgrading from 3.4 version to 3.6 version

[Mashxurbek_Muhammadjonov]

The problem solved. It appears I had to import a specific gpg key from ubuntu keyserver.(key: 2930adae8caf5059ee73bb4b58712a2291fa4ad5)
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2930adae8caf5059ee73bb4b58712a2291fa4ad5